The answer to that is no. Neither players nor GM is required to purchase subscription and all can play free, start a campaign for free (GM), and mix together subscribers and non-subscribers. Subscribing gets you some additional features-benefits in the program, and helps support development of Roll20. If the GM is subscribed, all the nonpaying players in that GM's campaigns will "see" certain benefits provided by the GM's subscription, no problem. The benefits are listed, of course, on the SUPPORT US link at the top of the page.