[quote=@Shienvien]
<Snipped quote>	In addition to interest check threads, marking the RP as "Apply" or "jump-in",or - if you have had contact with them and have the feeling they might be interested -  pinging them in the thread or even PMing them?
[/quote]
I mean being able to invite new participants to an ongoing PM thread of any kind, not necessarily one that has something (or anything) to do with an Interest Check or ongoing RP thread out in one of the general subforums.

For example, I am a co-GM in a game currently running in the Advanced forum. During a period when the game's primary GM was out of commission due to RL issues, I had begun a PM collaboration with one of the players, filling in for the GM on a longish post for the IC that was to be posted up all at once when completed. Without really thinking about it, I had begun the PM with only myself and the player as participants; when I then tried to add the primary GM as a participant so that they could participate if they returned sooner than expected, I found that I could not. 

Other PbP forums (and forums in general) that I've been a member of have had the ability to "invite" new participants to a PM thread at any point, and by any current participant of said thread, and I've repeatedly found this functionality to be useful, especially where PbP games are concerned. Additionally, some forums I've been a member of have allowed a participant in a PM thread to "uninvite" themselves, in effect removing themselves from the conversation (without necessarily deleting said PM entirely), and some have given the OP of a PM thread admin-like privileges that allow them to not only invite new participants but "uninvite" existing participants (perhaps because they're being disruptive to the conversation, or because inviting them was an unintentional mistake).

So I figured I'd bring it up here. :)