Users can be uploaded in bulk using the 'Bulk Users Upload' feature within the 'Administration' tab within AMP. This is a feature available for System Administrators. In order to utilize this feature, you can select the 'Bulk Users Upload' link from the 'Administration' tab for an organization.
Following the format specified in the 'Bulk Users Upload' page, upload a csv with the correct information. Your CSV should look something like this:
Once that is uploaded via 'Bulk Users Upload' and the CSV is in the correct format, the new users will populate in the list of users currently belonging to that organization.