In the Forza Dutch Supercar Challenge lobby, there are four player buckets and four teams.

The Player Groups are:

Unlimited--this is for all Supersport and Sport street cars, as well as all modified street rides in GT and all cars in Super GT that do not have a group designation (see Car Specs).  This group has no handicaps.

GT500 GT2 GT4--this one should be pretty clear; that is, all GT500 chassis competing in the Super GT class and all GT2 chassis competing in the GT class belong in this group.  All of the GT4 chassis competing in Supersport also go here.  This group has a -5% handling handicap.

GTS--the old '90s GT1 chassis competing in Super GT go here.  This group has a -15% power handicap.

Supercars--the monsters live here.  This group is reserved for the likes of the F50GT, the 599XX, and others like them.  Almost entirely made up of low-production hypercars, this group is for GT's and Super GT's most powerful cars.  This group has a -25% power handicap.

The Player Teams are:

Yellow:  Super GT
Red:  GT
Green: Supersport
Blue: Sport

There.  I think that should be simple enough.  To take part in any race, just sign up and make sure that your car is in the right group to compete, and you are on the right team, and you're good to go.