Grabbing a random example from F1 2011:
http://www.racedepartment.com/forum/threads/f1-2011-practice-japanese-gp-01-05-2012.50493/
Basically for a single race (a so called Club race) you make a new thread. You specify:
Date + starttime (in GMT, BST and/or CET) track, distance, banned assists (moreless default for clubraces on RD is 50% distances, all assists banned except autogears, and dynamic weather). Since car handling is quite unknown people may like a familiar track better, such as Spa for example.
People who want to sign up, can specify the car they prefer, and it's up to the threadstarter to edit his post to keep that list up to date.
Then the hosting thing. This one is a bit tricky because we don't really know what's gonna happen with hosting and if it will work the same way as previous F1 games. People probably are gonna test that as soon as the game is out. Most of the times a few guys hosted all the races.
In F1 2011 it was common that the host added everyone who signed up. Due to the migration to Steam, I think it is a lot easier if things go the other way around: Steam link to the host will be posted so everyone can add him easily. A steamlink to someones profile is either steamcommunity.com/id/<a name here> or steamcommunity.com/profiles/<a long number here>
Unlike GFWL, Steam nicknames don't necessairly have to be unique, and people can change their name if they want.