There two major types of groups, PUGs and guild groups.
PUG stands for Pick Up Group. Sometimes, anyone who is not part of *your* group is called PUGs. It can sometimes be used as a derogatory term by groups who consider themselves elite, though factually, any group that is not pre-organized as part of a guild is a PUG. PUGs are generally open to anyone, though some will require that you join in for voice communications. You never know what you’re going to get with a PUG.
Guild groups can be of any size up to a full 24 person group. These groups are usually restricted to guild members only. Sometimes they will be open to random people in order to find new potential members, though that is the exception and not the rule.
When joining a group you are expected to follow the directions of the leader. If you don’t follow the leader’s directions, you will probably be kicked from the group. You may not like the directions, and you may not believe that the leader’s decisions are the best or most appropriate ones, though you still need to follow them. If you don’t want to follow the leader of the group, then leave the group.