I didn't understand how gta online will work but here is how I think it is.
You can create a Crew with friends or join a public Crew. 5 crews max you can be in. There are over 700 missions.
There can only be 16 max in a world so idk if that means 16 is max in a crew.

Your crew can also fight other crews in deathmatch and gang wars in some kind of matchmaking. I don't know if R* has dedicated servers or will be leaving this up to hosts...