With SharePoint Foundation 2013, we seem to have a similar problem as described in this thread: http://social.msdn.microsoft.com/Forums/ur-PK/sharepointgenerallegacy/thread/b33a036a-c1bf-47b5-a95f-423985407e68
Since 2013 has moved to a javascript powered spread sheet entry mode, it seems that we cannot properly add a user with the people picker if there is a comma used in the user id. Does anyone know a workaround or if this is going to be fixed? It seems like it was a problem with 2010, and I can't believe that it was not addressed in 2013. Thanks!