Hi Krissy,
Usernames must be unique throughout all of ArcGIS Online. If a numeric value is being added during the invitation process, chances are that someone has already created the same username. The numeric value is added when the requested username is not available. For this reason, sometimes it helps to add your organization short url to ensure that the username is unique.
If you feel like this may be an error, feel free to get in contact with technical support to investigate the issue further.
-Kelly