Welcome to mystery shopping and the forum!
Market Force has another client that allows you to take a guest. I can't name it here in combination with the company. That would be violating our ICA and a rule of the forum. You can search the job board there.
There are some shops or types of shops that usually allow you to take guests. Sit-down restaurants, (sometimes called fast casual, not fast food), are a good one. Most sit-down restaurant shops in fact require a guest because they require two entrees to be ordered. There are also museums, amusement parks and others with admission tickets to be purchased.
Good luck and have fun with it
Happily shopping Rhode Island and nearby Massachusetts and Connecticut