Board Thread:The Quarters - Q & A/@comment-173.51.205.128-20140416192554/@comment-24150802-20140416200137

Did the User IDs that you use look something like 3b7d59d1-9c79-4c96-a9ac-6e01664d4251? If not, you might have been using their display names, which is a different thing, and which won't work for invitations (because display names are not unique - several different people could have the same name). You can see instructions for finding someone's User ID here on the wiki.