How can I require multi-user approval for a node?
Maybe write your own module

#1 push message for each team member

#2 for each member approval check if this it's all, if yes then push invite to new member/user

#2b if new member accept invite then add her/him to the group (you can do it with og API e.g.)

#3 if someone from group disapprove then remove invite from awaiting list

#x you can add some settings page so e.g. if 50% of team members agree with invitation then proceed to #2 and invite new user