I've not joined a league yet but most are played on the official Prem League site (same as we used for the Euros) so you can always set your team up now and join a league closer to the time. It's the same set-up, 100m with unlimited transfers up till the first deadline. players will obviously change as transfers take place but you can at least work on building some kind of spine to your team. Good luck.
http://fantasy.premierleague.com