Why did they meet up. Well there's a host of reasons I can think up for sure. Basically they fall into three categories though.
A. Dumb luck and sheer coincidence
B. Known ties between the players
C. Unknown ties between the players (but naturally known to DM)
So by category I can think of the following one-liners why they would group:
A. Dumb luck
1. Everyone happens to be at some gathering place (think inn, marketplace, harbor) when some event (think someone asking for people for hire, a discussion or brawl of kinds where the PC's agree with "their" side) causes them to become a team.
2. They're the only survivors in some "disaster" (think ship wreck, town slaughter, caravan robbery).
3. They were seperate smaller teams or pairs maybe (formed by any of these reasons) and ran into each other en-route to some well known adventuring site and decided to group up.
4. They were all "picked" by some evil being(s) (think captured by robbers, beslaved by bugbears, used as gladiators in some barbarian city) then they fight out as a group with what little resources they have.
5. They were grouped together by a superior and thus became a team (patrol in some nations or mercenary force).
B. Known ties between players
1. They all share some kind of relative with each other making them (possibly distant) family to each other (brothers and sisters, nephews and nieces, father/mother and son/daughter you name it).
2. They all grew up in a certain area (neighborhood, farm village, woods) and have been a "gang" since childhood.
3. They all shared a common interest or place to visit (school, a job at a farmer, fighting school, the arena, stable boys in the royal stables) since childhood and were always close because of it.
4. They all strive to aid the same deity, church or movement and were made a team in that way.
5. They are all of the same race and are (probably) the stoutest and boldest of the younglings thus being banded together.
C. Unknown ties between players
1. They all have a (probably far) relative and for some reason (revenge, a "taste" of the far shared relative's power through his blood, wiping out the bloodline to prevent them to get some right from the relative (wealth, power what not) so said person gets the benefit.) wants to slay them all causing them to band up.
2. They all have been sent there by some superior (deity, king, commander) to investigate and bump into each other there seemingly dumb luck.
3. They all have gotten in possession something (necklace with an odd piece hanging from it, rare ingredient inside some kind of jewelry, piece of lore or knowledge that is useless individually) somehow (stolen, inherited, bought, traded, found, obtained through killing) that together make a special thing (powerful evil item, key to some ancient evil place, ancient evil spell) causing the evil person to want them all together bad hunting them all.
And undoubtedly a ton more but this is what I could come up with on the fly. Actually the last one's might work with your evil dude lures them all to the same spot. If you don't make the tie too obvious that should work out. I mean if you have them write "1/6'th of a piece of parchment" on their sheets the 6 of them will have figured out in no-time what the tie is. But if they each have their "own items" and you just decide it's in there without them knowing that might keep it up a while. Then spill some hints like the evil guy saying "Lay down your belongings and you may go freely" and so on giving them hints that he's after something they have. Might work.
Erky
A. Dumb luck and sheer coincidence
B. Known ties between the players
C. Unknown ties between the players (but naturally known to DM)
So by category I can think of the following one-liners why they would group:
A. Dumb luck
1. Everyone happens to be at some gathering place (think inn, marketplace, harbor) when some event (think someone asking for people for hire, a discussion or brawl of kinds where the PC's agree with "their" side) causes them to become a team.
2. They're the only survivors in some "disaster" (think ship wreck, town slaughter, caravan robbery).
3. They were seperate smaller teams or pairs maybe (formed by any of these reasons) and ran into each other en-route to some well known adventuring site and decided to group up.
4. They were all "picked" by some evil being(s) (think captured by robbers, beslaved by bugbears, used as gladiators in some barbarian city) then they fight out as a group with what little resources they have.
5. They were grouped together by a superior and thus became a team (patrol in some nations or mercenary force).
B. Known ties between players
1. They all share some kind of relative with each other making them (possibly distant) family to each other (brothers and sisters, nephews and nieces, father/mother and son/daughter you name it).
2. They all grew up in a certain area (neighborhood, farm village, woods) and have been a "gang" since childhood.
3. They all shared a common interest or place to visit (school, a job at a farmer, fighting school, the arena, stable boys in the royal stables) since childhood and were always close because of it.
4. They all strive to aid the same deity, church or movement and were made a team in that way.
5. They are all of the same race and are (probably) the stoutest and boldest of the younglings thus being banded together.
C. Unknown ties between players
1. They all have a (probably far) relative and for some reason (revenge, a "taste" of the far shared relative's power through his blood, wiping out the bloodline to prevent them to get some right from the relative (wealth, power what not) so said person gets the benefit.) wants to slay them all causing them to band up.
2. They all have been sent there by some superior (deity, king, commander) to investigate and bump into each other there seemingly dumb luck.
3. They all have gotten in possession something (necklace with an odd piece hanging from it, rare ingredient inside some kind of jewelry, piece of lore or knowledge that is useless individually) somehow (stolen, inherited, bought, traded, found, obtained through killing) that together make a special thing (powerful evil item, key to some ancient evil place, ancient evil spell) causing the evil person to want them all together bad hunting them all.
And undoubtedly a ton more but this is what I could come up with on the fly. Actually the last one's might work with your evil dude lures them all to the same spot. If you don't make the tie too obvious that should work out. I mean if you have them write "1/6'th of a piece of parchment" on their sheets the 6 of them will have figured out in no-time what the tie is. But if they each have their "own items" and you just decide it's in there without them knowing that might keep it up a while. Then spill some hints like the evil guy saying "Lay down your belongings and you may go freely" and so on giving them hints that he's after something they have. Might work.
Erky