Greetings Mr X . All you have to do is put an entry on any of the threads that you are interested in, expressing that interest. If there is space available, you're in, as simple as that. Even if the race/series is fully booked up, there are often cases where people drop out, so it's usually possible to sign up as a substitute.
The Radical Racing event is nearly finished, but if we have some space for next Monday I would imagine that you would be very welcome.
(And the Road B three week series is bound to be a real belter. The guy who's organising that is one of the best )