You can use the getLiveRoomInfo() method which will return the list of joined players along with other info for e.g. room properties
http://appwarp.shephertz.com/game-development-center/corona-client-apis/#getliveroominfo
Result of the request is provided in the “onGetLiveRoomInfoDone”
'joinedUsersTable ' of roomTable will be containing the usernames of all joined players