@pflstr from #5
In my opinion, there are several questions to be answered.
a) Who can add or remove recipients of a private conversation? If it's only the admin it's quite easy, we just need to add a special case for a recipient wanting to leave a private discussion. This right will be given in addition to the right of the admin.
b) If the right to add and remove recipients of a private conversation is given to non-staff usergroups as well, one should be aware, that a breach of confidence might result, if posts older than the time of the change of recipients become accessible to new recipients. This needs to be avoided to protect a forum owner from legal implications. The right to leave such a discussion voluntarily would apply in this case as well.
c) If users are allowed to leave and reenter a private discussion, we may need more than one entry date and / or more than one leave date. The database query to filter out the posts that should be shown to such a user may become quite complex. What should be done with quotes from posts, that are not supposed to be shown to a certain user?
My suggestion: Don't let the list of recipients be altered after the start of a private discussion: The only exception should the case of a user leaving such a discussion voluntarily. Even in this case one should favor a solution, where the given recipient is not removed from the list but instead the discussion can be ignored by any recipient (and the ignoring can be revoked without legal consequences, because the user never really left).
If participants of a private discussion want to expell a certain user, they can simply start a new discussion without the given user.
One last question remains:
d) Who should have the right to close a private discussion? The original poster? Any participant? And who should have the right to reopen a closed discussion?