register the database with the Copyright Office (assuming it is protectible -- see Circular 65), require that users enter into a click wrap arrangement which grants them limited rights and guarantees your rights to go after those who copy data without authorization, andtreat the database as a trade secret and require that employees and contractors working on your site enter into nondisclosure agreements.
Can You Really Prevent Theft?
These legal measures provide tools for chasing down thieves and recovering damages. They may discourage theft but none can prevent it. (Websites can be 'scraped' and databases copied.) As for getting the proper language for your click wrap, start by looking at click wraps for similar sites, or by checking Steve Fishman's book.
Get a License?
Yes, the owner of the database should enter into arrangement with the other partners. A license makes sense because you can contribute the database to the partnership and still retain copyright ownership. You may have to determine the value of the license to assess your partnership contributions and partnership taxes. An accountant's help may be necessary. (Check out this Nolo book or software program.)
What's the Picture Got to Do With Database Protection?