I'm not sure to be honest, as I haven't done any of the admin, but here's where you would register a team -
https://wtrl.racing/ttt/TTT-Registration.php (you may need to register/set up account on the website to do this - you need to in order to see the reults etc).
Check out the rules section too, it will have all the details about what riders you're allowed in the different categories.
A couple of things to note - all your riders need to get the EXACT same wording tagged onto their name on Zwiftpower so the organisers can pick up the team results - e.g mine had to be "Paul Bridgman (ZSUNR - Jupiter)"
Also note that the category pen you all join for these races has nothing to do with your individual categories - you'll be told which Cat pen for all your riders to join (e.g. we had to join the D-Cat pen last night). You'll also be given a start time which will usually be a few minutes after the clock gets to zero on the start line - a bit like our handicap meetup races. Last night it was 2 minutes we had to wait but it has been as many as 9 minutes. So you'll need someone to be on the case with a stopwatch!