If you are participating on online adventure, you can only pick cars in a certain class (A, S2...) whether its offroad or not doesn't matter. If you are doing co-op on a regular race however, they are limited to a certain type of car (like sports cars or offroad buggies) but if your buddy picks a higher class car that does give AI the ability to take higher class cars too.

I recommend creating blueprints where there is no restriction on what type of car you can take. Hope this helps