Sites are added into placement groups as placement requests. This can be done manually by university Placement staff or by the sites themselves.
See "Sites updating their placement requests" article for how the sites add placement requests themselves.
The first attached guide gives instruction for how university Placement staff can add placement requests to placement groups both individually and in bulk.
The second attached guide shows how to use the Offers module.
Here is a video demonstrating adding individual placement requests to a placement group:
Here is a video demonstrating adding placement requests to placement groups in bulk: