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You don't actually technically have to tag them, just talk to the person and tell them about the thread. There isn't really a set system for 'adding' people to Private RP, it just sort of happens.
Don't put them in the Tagging area of the post. In the post, where you type where you talk etc, you @Mention
It's more called Mention than Tag, generally. Like right here where this text is, up above where your name is put in my post. That's where you mention them. They'll receive a notification they have been tagged.
There's nothing that really keeps anyone from posting in the private threads except social norms. You don't need to tag or add them somehow, they just need to know they can post there. Of course, tagging them with the @mention or the [ member=" person" ] does help them keep track.