There are certainly a lot of RVers and 'snowbirds' who do mystery shop. I am in a very big 'snowbird' area and available work noticeably declines as they arrive. A couple of my schedulers have gotten the message that if I am not continuing to be awarded shops during 'snowbird season' they will also need to find someone to do them once the snowbirds are gone or pay hefty bonuses for summer work. A number of RVer magazines apparently have articles on shopping as a fun hobby to do when you arrive in a new area and have time on your hands.
There have been many discussions of encouraging friends to shop and most of them end with 'I wish I never had'. You don't really know the work ethic of your friend. You don't really want to be their hand holder and job supplier. You don't really want to lose them as a friend because they got shops rejected for not doing what they were supposed to do and getting mad at you for the money and time they lost.
I have never referred any person for a fee. I find mostly the companies that offer fees are not particularly companies I would refer folks to. I do refer fellow shoppers I have come to know on forums when a scheduler asks if I happen to know someone in ____. These are done after contacting the shopper friend to see if they would be interested. I have found that it works well in improving the relationship with the scheduler and if it works for a fellow shopper, that is a bonus in and of itself.
I have also sent a family member to do a shop I was out of rotation for when a desperate scheduler has called. We quickly get them signed up to shop and I coach them and get them set up to go do it. They get paid through their own shopper registration, I get a grateful scheduler and I get more respect from the family member for what I put up with.