Join and Leave via REST - howto?

In theory I think the way to do this would be by creating a group membership via the API. Unfortunately I think https://www.drupal.org/project/group/issues/2872645 🐛 Creating `Group` content is impossible via REST, JSON:API and GraphQL due to `$context['group']` being required in checking `create` access Needs work would prevent that. Though someone more knowledgeable about the module's internals might wish to jump in and correct me - I am not a maintainer.

There is an MR attached to that issue. I have never tested it, but it'd probably be worth looking at. I think if you did have the patch you could do something like this with jsonapi to add a group membership:

curl --location --request POST 'https:/cms.foo.com/jsonapi/group_content/subsite-group_membership' \
--header 'Accept: application/vnd.api+json' \
--header 'Content-Type: application/vnd.api+json' \
--header '<omitted>'  \
--data-raw '{
    "data": {
        "type": "group_content--subsite-group_membership",
        "attributes": {
            "group_roles": [
                "subsite-group_administrator"
            ]
        },
        "relationships": {
            "gid": {
                "data": {
                    "type": "group--subsite",
                    "id": "a6aaabbb-2e59-40c3-b7d2-367e66a4d6a5"
                }
            },
            "entity_id": {
                "data": {
                    "type": "user--user",
                    "id": "d19e7446-4b89-4884-9acf-d921e21edc12"
                }
            }
        }
    }
}'

Thank you very much! I will have a look ...