Can I Invite Specific Players
You can rMail specific RPOL users to invite them to join a game, yes. I think four of the last five games I've joined happened that way. However, using rMail to advertise your game, spamming people with invitations to join, is a fast way to get the hammer dropped on you (it doesn't happen often...and I still remember laughing to myself the day I got a message like that, only to see a few minutes later that the same RPOL user had also included BigBadRon on his spam list--when I read in the Chamber of Lost Souls about how he'd been banned for spamming with game invites.)
If they are people you already know, you're fine. If they are specific players that you're interested in playing with, you should be okay. You might have to set the 'Accepting Players' flag on the game, but you don't have to advertise it in Wanted: Players, and as soon as your friends have joined up, you can shut off the flag (and you're under no obligation to accept anyone who might stumble across it while the flag is set and sends in their RTJ...)