[@Mahz] Thinking about it, can you not recycle the coding you're already using for the "[+ Create Convo]" button? Obviously that's [i]creating[/i] a PM thread at the moment, but excising that portion and instead only keeping the bit that INSERTs the inputted usernames to the specified PM thread's list of participants seems like it would work fine. Then just rename the recycled button "[+ Invite]" or some such, and there you go.

If not, it seems like a relatively straightforward stored procedure could be written (assuming you're using T-SQL) and then called by the hypothetical "[+ Invite]" button using the Java.SQL.CallableStatement class.

Of course, knowing as little as I do about both your database schema and how you've structured the site's coding in general (by which I mean, "pretty much nothing"), I could be grossly simplifying the problem here.... ;)