>>Is there a way to add all users to a product?< <
Yes, there is. On the Users > Manage product, do a search, and then click on the “Select All” check box, select all users, then select the product you wish to add them to, and click on “Add”.

>>Or to multiple products?<<
You can select multiple users, but cannot add them to multiple products in one shot. You will have to add the users to one product at a time.

- Ravi Jayagopal