With the twitch.tv/membership capability you will receive a NAMES event on channel join, and JOIN, PART and MODE events while in the channel. Requesting the userlist manually via the NAMES command is not supported.
Remember that the userlist is cached and can take it’s time to be received, these events are not instantaneous like they would be in regular IRC. On some channels with too many users you may not receive one at all (unless that changed).