Have you tried to add users to your ArcGIS Online subscription and have been shown the error message "Unable to invite the following users"? One of the reasons for this could be if the user has an apostrophe in their username.
Unfortunately, at the moment ArcGIS Online isn’t able to support the creation of accounts when users have an apostrophe in their email address. When you try to add these users you will see the following error message:
However, this is a bug which is going to be addressed in future releases of ArcGIS Online. For now though the workaround we recommend is creating an email alias without an apostrophe, and then sending the invite to this email address.