Ensure all members of clan have the rest of the clan on the friends list.
Add one member of opposing clan to friend list.
Initiate private game.
Invite your clan members and opposition member from friends list.
Wait for opposition member to invite his team from his friends list.
Hope everything works and the connection is half decent.
And how do you counter the 0 ping host advantage?
Especially as you do not control who hosts so cannot split it evenly between the teams.