I think a variation of this that could possibly work is to expand the free trial codes to something that could give existing TR subscribers a limited number of group workouts that they could invite non-members to.
Maybe something like 1 or 2 invites a month. This would allow members to occasionally do some rides with non-members, and would potentially get new people to try the platform out. I can see people who are willing to try a group workout that won’t bother with a free month.
This doesn’t help the OP’s use case of group rides, but I could see something like this be manageable from a business/technology point of view.