Open the Residently app and go to Support > Create new message > Request changes to your tenancy. Enter your request to add, remove/delete or change a resident, guarantor or permitted occupier, and the property manager will be able to make the changes for you.
The number of guarantors you can add will need to be discussed with the property manager. Adding more guarantors than previously agreed needs to be communicated directly to the property manager via the Residently app using the routing options above.