If there is an organized military, religion, kingdom, that the group belongs to, they can simply be hired based upon their reputation. This requires the perception of the campaign that level 1 PCs are already relatively prominent in local areas, or start higher than level 1.
I typically prefer to do things organically, bringing in the character's own motivations and trying to weave them together. This is most definitely not simple, but it has the best payoff if you're patient for it.
Probably the "easiest" way is to just tell the players that they need to be an established group. This can force players to work together before they even create their characters, or try to figure out a way to get each other connected. Some systems encourage this, using "Bonds" or "Connections" or "Associations" to make it easier.
I usually start my campaign in a relatively large city or populace center, which makes it easier for multiple contexts/desires/goals to meet up in the same place. I can generally get the entire group together and pretty connected in one session.
For example, my last campaign (Star Wars Saga) I let everyone create whatever kind of character they wanted as long as they were aligned neurally or better, had no extreme political views, and that they were on Coruscant. They were all notable in their own way, one was a decorated war hero, one a well known smuggler during the last war, one a Jedi looking to prove himself after a failed mission, one a junior senator, and one the son of the ambassador of newly recognized planet in the senate.
They were all invited by an individual who believed they could be of use, along with around 150 others. Of course, the man who invited them to the party had some people looking to kill him, one thing lead to another, and bam the party turned in to a firefight. Instant friends and colleagues.