They're a huge company and have been in the business for several years, so I would trust them. But in the end it is upto you whether you want to give it to them.
Yes, CPA networks pay you money on the work that you do for them. You need to supply them with either a taxid or SSN# so you can get paid.
Have you ever fill up job application without entering your SSN? Did you feel like your signing your life away?
your going to HAVE to give them either a SSN or an EIN number for tax reasons. If you are uncomfortable with disclosing your SSN, just register your business and get an EIN number. There is not much you can do (fraud Wise) with an EIN number.