Lists the member ids currently subscribed to a presence channel. Ids only: `member_info` (the profile data attached by your authorization endpoint) is delivered to subscribed clients over the realtime connection and is not available over REST. An app-defined member id — the identity of your application's end user ("member"), assigned when your auth server authorizes them. Never a Bird user. Max 128 characters, restricted to URL-safe characters because member ids appear directly in API request paths. Broader than a channel name — allows `+ : @ . _ -` etc. for real identifiers (phone numbers, emails, `member:42`), but excludes `/ ? # %` and whitespace.