@Robert Walker
I pretty much did what you did Robert..
I did both too, got a email from simracing.gp Asking me to confirm my account.
( cannot remember if that sent me to beta.simracing.gp )
I then went to beta.simracing.gp, that asked me to use my Steam account to log in.
Steam then sent an e-mail to me with a conformation code, which I put in.
I then ended up in the simracing.gp with a list of organisation,
I picked Racedepartment,
then from the list, pick Assetto corse, RD historic Racing club.
in this case there is only one AC race, scroll to the bottom,tick the car,.
then It should show one less participant, ie you.
refresh page, you should be there at the bottom.
this is how I muddled my way through it, rightly or wrongly.