The value is based on valuation by people, which is basically decided by asks and bids on exchanges. So yes, buying and selling is a big part. Of course part of that is perceived value, part of that is amount of actual use and part of that is usually temporary market manipulations. New users can definitely help the value of HIVE grow, although not in the same amounts as e.g. BTC because of the differences in token generation (HIVE is basically constantly produced).