This module will help you to migrate existing users from a different platform into HumHub. You can simply import users to the database from a csv file and notify them. Mass User Import provides also a REST-API Interface.
After installation and activiation from the Marketplace go to Administration -> Users. You'll find there 2 new entries called "Invite Users" and "Import Users". Then follow the instructions there.
To use the REST-API interface you first have to activate it in the module configuration. The you find the api documentation next to the "Invite Users" and "Import Users" entries. To access the REST-API you need to specify a API-Key.