Group type is default on schema nms:recipient. If you wish to add someone or remove someone from Group dynamically you can use the API methods Register and Unregister
In case you are working on a LIST which allows you to group records from a different schema, something including other than nms:recipient and is generated from a workflow you don't have an API method available out of the box. Schema used is nms:rcpGrpRel and since the targeting dimension can vary it is difficult to define an API method.
A possible workaround can be to add such records or PK value to a schema like cus:deleteFromList. You can then define a workflow that runs every 15 minutes, checks for records in this table and then delete the connected record from list. After that the original record from this table cus:deleteFromList should also be deleted.