Which public transport ticket is needed?

Either you join a 5 - group ticket, which costs 5,10€ p.p. per day or you take care of a BVG ticket for the AB area yourself. https://www.bvg.de/en/subscriptions-and-tickets/all-tickets

Can I bring someone without a ticket?

No, you need a ticket and pay the participation fee. If you want to bring your partner or a contact from the START Network, please clarify this with the organization team beforehand.

Do you also offer train DB group tickets from Munich to Berlin?

No. These are not financially worthwhile for people up to 26, because you always travel cheaper with a Sparpreis. You have to organize arrival and departure on your own. We recommend you to book a train with Deutsche Bahn with BahnCard 25 on Wednesday.