I don't think there is any method presently available in construct to do this. The best you can do for the moment is have an room name list (dictionary) that the game cycles through joining each one until it finds one that isn't full. the host detects how many players are connected and when enough have logged in the game starts.
Other wise you would need to have your own website and working database with player logins, ids, room creation and game management to handle players that want to create, set room passwords and join rooms (that's what I'm currently setting up for my own games). Unfortunately this requires a pretty good deal of knowledge regarding web development and database design (its only a hand full of tables but the logistics is somewhat tricky).
However It also prevents me from running the game as a standalone App with out building the room creation/joining into the game (which im avoiding becuase that would be really difficult). So my games for now will all be purely web based to maintain session info and keep players synced to there rooms. My biggest fear though is the by the time i get it all up and running Ashley will have created some amazing plugin that will handle this and my work will be for nothing... for now though to my knowlede nothing is available to manage room creation/joining