Forum Invitation Can Not be Accepted if Forum Already Added
When a user shares a forum with another user that already has this forum, the invitation can not be accepted and thus forum posts will only be received by the initial inviter.
This can be a problem if the initial inviter is not online frequently or even deleted their account.
Ideally, invitations can be accepted by more than one person, so more people can share forums with each other making for a tighter mesh and faster sync of forum posts through the network.