Create a recipient group

Creates a group and, optionally, fills it in one call. Repeating the call with a name that already exists returns the existing group with 200 instead of creating a duplicate; the members of that existing group are left exactly as they are, so use the update endpoint when you mean to change them. One caveat for retries: the plan's group limit is checked before that name lookup, so retrying a create that already succeeded and filled the last free slot answers 429 rather than 200 — treat a 429 on a retry as "probably already created" and check the listing.