This is actually local to here as it works on my RC2 installation. I create a new conversation, invite someone, and proceed as normal. I do not allow anyone to add people to the conversation (meaning only I can) and I do not lock the conversation. When I'm viewing the conversation, I click on Invite More, type in other users, and then click Invite Members. The form submits, the Conversation Participants block refreshes but the person isn't listed nor are they invited to the conversation.