Check out this web-site that sites a whole lot of companies that do wine tours: http://www.wine-tours.it - here you will find a list of wine tour companies and their web addresses.
For private tours I can recommend Franco firstname.lastname@example.org or Cristian and Azzurra from Viaggi d'Ambra.
Joinable wine tours are done by several other companies such as Italy & Wine, Angela S. & Tuscan Trails, and so forth. I don't have personal experience with either of these, but maybe you could forward the question to others who might have taken another joinable wine tour.
Good travel forums to ask on are Tripadvisor, Fodors and Rick Steves.