360Alumni utilizes a "User's Email" address as the unique identifier for account creation. As such, if you are a member of multiple 360Alumni client communities; you will need to use a unique email address for EACH account that you activate.
For Administrators, you may also run into this issue when importing user data into your 360Alumni community database. If the email address associated with a user record is already in the 360Alumni database (for your environment or a separate 360Alumni client) you will not be allowed to associate this email address with this user. These exceptions will be reported back to you, and you can attempt to include a different email address or simply remove the email address associated with the record.