If I type in Flo and clicked "Floris" on the auto-complete, the text cursor stayed after the "o" where I stopped typing to use the mouse to click on the name, rather than the text cursor being after the comma of "Floris,".
Also, if I click "Invite More" and after adding several people, the lightbox looking invitation box won't go away. Instead it will still display the "add users" pop-up with the buttons "Invite members" or "Cancel", making me think that if I hit "Cancel" that my previous people added will go away.
Perhaps either make the window go away after inviting them, or having a "confirm" button that closes the window after you're done, so you may go back and continue a post. I'm not sure if this may be the cause, but I had typed about half a post to a private convo before inviting the people, whereas I was going to finish my post before clicking submit.