It is almost impossible to determine which MS companies represent any particular restaurant or hotel. The only way to find out is to sign up with as any companies as you can. With regard to hotel shops, sometimes they ask you to make a deposit on your credit card up front, to insure you will actually conduct the shop and submit the report.
How do you get paid? Just like with any other job, once you submit your report for a shop, the company will pay you at the end of the month or the following month. Many companies use Pay Pal, which works great. Others use direct deposit or mail you a check directly.
Jan has a list of legitimate companies on this website. Just read some of the other posts here. You will not have to pay anything to register with the companies on the list. They are all begging for mystery shoppers. Some of them even pay a $50 bonus if you refer another shopper who completes 8 hours of work.