Idea #1:Group Leaders and Group Closing: This allows whoever started the group to lock other group members from joining friends into the group. Also, Group Kicks that allow you to kick someone out of a group. Idea #2: Group Lockouts: This will make it impossible to join someone else's battle, go into a dungeon with them, or teleport to them unless you are in a group with them.
I think your group aura suggestion is very useful. It would end the trouble I have at big dungeons/bosses, where tons of people crowd around the sigil hoping to join without an invite.