Red Dead Redemption 2

This is pretty easy but the wording can be a little confusing. You don't HAVE to be the Posse leader, and as long as you stay in some Posse that's actively doing stuff you'll get it eventually. However, if you are like me and prefer to be in control:

1) Start a posse, it can be temporary or persistent, but go persistent if you can afford it (200 dollars plus your camp costs) since it'll pop another achievement anyways. Make it open instead of invite only to better your chances of getting members. Alternatively, if you are doing a session with people you know and need this achievement, you can limit it to them since it'll pop for everyone in the posse if they don't already have it.
2) Once you've created your posse, either wait for members to join or send out invites. Note that there is a limited number of invites to send out at once.
3) You can continue as long as you have at least one other member, but I prefer to have at least two in case someone drops out.
4) Go to the nearest Stranger and accept/complete their mission. As long as at least one other posse member was with you when you completed it, achievement will pop.
NOTE: you can't accept a stranger mission if you're not the posse leader, which is one more reason to do it this way if you want more control.

There might be easier ways of doing this, but I never got it to pop on natural progression and I got this really easily and right away after I had gotten the achievement for created a persistent posse.