Like a lot of people I tried and tried and tried as soon as Online became available with virtually no success. The Race trigger marker just disappeared. Then I took a few hours break to catch up with other stuff. Not much had changed when I went back but i persevered. Eventually, the first Race started (with just me and Lamar, lol) and from that point on there have been hardly any issues with the servers (sometimes it fails to recognise the Social club link and it says I don't have a crew but otherwise, all good),
A friend and I played for a good few hours with less than a handful of server-related problems.
Once the Race part says "joining session" if it doesn't pop you in within a few seconds, go back to dashboard and try again. You only need to get through the race and it is relatively plain sailing from there.
TL;DR Keep trying the race, eventually it will work and soon after that it all opens up and you get away from the first race "bottleneck".