Escargot's group chats allow you to open up a persistent, private chat that you can invite others to. Members in these group chats can have the roles of either an owner, administrator, or member. Anyone can view the list of accepted users and leave the group at any time.
Owners and administrators can also invite and remove members along with viewing the list of members pending approval.
Currently, the only version that supports group chats is Windows Live Messenger 2009 (MSNP18). It supports the creation of group chats, viewing the member roster, sending invites to others and accepting/declining them, sending and receiving messages, blocking group chats, and leaving them (so far this has only been confirmed for members that aren't owners).
However, the ability to manage existing members isn't natively supported by that client, so you'll have to log on here to do so.