Retrieve a single group by ID

Returns the full group record for the given internal group identifier (certifyGroupId). Use this endpoint when you already hold the group's certifyGroupId and need its complete detail; for lookups by NPI, TIN, name, or affiliation use GET /groups with the appropriate filters, or GET /groups/search for autocomplete-style name/NPI/TIN prefix search. The tenant-id header scopes the lookup to a specific tenant.