Group invitations needs to be properly implemented

[Nick Chadwick]

At present, group invitations are simply a flag on group membership, which
is not currently used.

It would be better to create an invitation object, which can be sent to
other users. That user should then be able to accept, at which point they
are added to the group (rather than having them be added to the group
immediately).

It would also be nice for there to be a way for inviters to see the status
of their invite (accepted, declided etc.), as well as have some mechanism
to stop multiple invites being sent to the one person, or being sent to
someone who has already declined a previous invite to join the group.

This could also be enhanced with the use of 'requests to join' in addition
to 'invitations to join', allowing users to ask to be a member of a group.

Finally, allowing groups to have their own private forum space to
facilitate teamwork would be nice, and be a good use of phpbb