I run a West Marches style game, but rather than having a lot of players, I have a lot of characters. Each player builds 2 characters at a time, and can't build any more until both of them have seen play (or all characters but 1 are killed).
I give the adventure introduction, and then the players each choose a character. If the party has less than 5 players, the group may allow someone to play another character as a henchmen. They do not RP the character, and they only earn half xp value (but take up a full share). Henchmen make it easier to complete adventures at the cost of an overall reduction of the amount of xp, so the option isn't always chosen.
I give the adventure introduction, and then the players each choose a character. If the party has less than 5 players, the group may allow someone to play another character as a henchmen. They do not RP the character, and they only earn half xp value (but take up a full share). Henchmen make it easier to complete adventures at the cost of an overall reduction of the amount of xp, so the option isn't always chosen.