Can my fellow passengers simply join me on the regional day ticket for Bavaria?

Yes. You can use your regional day ticket for Bavaria as a low-cost group ticket. Your fellow passengers can join you on the way.

However, the number and identities of the people who travel with you are already determined when you purchase the ticket. You therefore cannot change your travel companions subsequently.
Passengers must show a valid ID document when asked to do so during the ticket inspection, in order to confirm their identity.