"The very first thing that strikes you is, that there is no Lobby to join up with others which makes the game seem much more user-friendly in many ways. If someone is forming a posse, and you're on their friends list, you'll get an invite to join. Accept or decline, it's up to you.
It won't affect how or what you're doing if you do decline. You'll be given a meeting place, and if you happen to be over the other side of the map, which is HUGE by the way, the host has the ability to – well – teleport you to the location where you can meet up with the rest of the posse."