This same problem was yesterday with WTCC.I don't know why people did't join with R3R.Maybe it's too expensive , and maybe lot of people did't know that multiplayer run very well.
Sadly completely out of my realm of control though.
Personally I think the reason is rather the cost of buying content for R3E combined with the fact that we are quite spoiled for choice these days with several other sims to race in.
@Pär Öqvist that's nice that they are having a free weekend. I will look into the track more and see if it would be feasible for this race and if others are interested as well. Also which car in the P1 slot did you want? DMD P20, DMD P21, Mistral M530, Mistral 531