Tool: Pyflakes
Processed Files:
    reviewboard/webapi/resources/review_group_user.py
    reviewboard/webapi/tests/test_review_group_user.py



Tool: PEP8 Style Checker
Processed Files:
    reviewboard/webapi/resources/review_group_user.py
    reviewboard/webapi/tests/test_review_group_user.py

"many-to-many"

"have to manually"

So as mentioned on Slack, this will result in an extra query per result (the Group.objects.get). It looks like the reason this is being done is so it can chain up a list of parents, which is fine in a general sense but not necessary for groups (which won't have a parent -- Local Sites don't count, since they're not part of the resource tree).
We have a few things going for us that the default get_href_parent_ids doesn't have. We know the value used in the URL, and we know the key needed for looking up that value and returning it. We also know there isn't a parent in ReviewGroupResource. So we can short-circuit a lot of the default implementation and do our own thing.
The default implementation doesn't know which ID we need, and it needs to handle any number of parents and corner cases involving the IDs. So it's going to do more work. It's going to check if the parent resource has a model_parent_key, and if set, it will fetch the object (much like you're doing here) and try to gets its parents (ReviewGroupResource will be a no-op here, since it doesn't have a model_parent_key). Then it will populate an entry in the ID map for parent.uri_object_key -> parent_obj[parent.model_object_key]. In the case of a Review Group, that'd be group_name: child.group.name.
But since we already know that group name, and are building a specific implementation (as opposed to that general implementation), we don't need to do any of that work.
I think this whole method can become:

parent_id_key = self._parent_resource.uri_object_key

return {
    self._parent_resource.uri_object_key: kwargs[parent_id_key],
}

Tool: Pyflakes
Processed Files:
    reviewboard/webapi/resources/review_group_user.py
    reviewboard/webapi/tests/test_review_group_user.py



Tool: PEP8 Style Checker
Processed Files:
    reviewboard/webapi/resources/review_group_user.py
    reviewboard/webapi/tests/test_review_group_user.py

 undefined name 'parent_id_key'

 undefined name 'parent_id_key'

Tool: Pyflakes
Processed Files:
    reviewboard/webapi/resources/review_group_user.py
    reviewboard/webapi/tests/test_review_group_user.py



Tool: PEP8 Style Checker
Processed Files:
    reviewboard/webapi/resources/review_group_user.py
    reviewboard/webapi/tests/test_review_group_user.py

This should check the response for stat=ok, item count, and the expected user.

Tool: Pyflakes
Processed Files:
    reviewboard/webapi/resources/review_group_user.py
    reviewboard/webapi/tests/test_review_group_user.py



Tool: PEP8 Style Checker
Processed Files:
    reviewboard/webapi/resources/review_group_user.py
    reviewboard/webapi/tests/test_review_group_user.py

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