I just started dipping my toes back into Mystery Shopping after a ten year hiatus. Lots of companies had been purchased by others, gone defunct, and changed platforms. New companies have also popped up.
To start with, I used the Prestomap to identify MSC's near me and signed up/updated those first. I also registered on Jobslinger and used the job listings to identify companies near me, then registered for those.
After getting all the ones close to me, I then used the Jobslinger connection system to register for the companies I had yet to register with. Next I went through the companies listed on the MSPA website and signed up for them in spurts. Then I went through the list here and registered for the stragglers.
At this point, I think I have registered for every company (My tracking spreadsheet has almost 200 rows). Most of these companies I may never work for, but some of them that were not in my area before now have shops in my area. In addition, shops are not posted all at once, and the favorites are gobbled up real quick. Example: I've been searching for a Jiffy Lube shop in my area and found it at a company that I didn't expect. It disappeared within 24hrs.
All in all, it probably took me 6 weeks to register for all the new companies and update the old ones, but if you sit down for 30 minutes a day and just do it, it really doesn't take that much effort. I use Lastpass as a password manager, and it also has a form fill option. Makes signing up much easier. Besides, if you want to discover which companies offer those exclusive travel/luxury/etc shops - it's probably the only way you'll find out.