What's a good idiom to describe someone who is included in a group only because they have something the group needs

If the word ticket is needed than surely you are looking for meal ticket.

Informal: someone upon whom one is dependent for one's income or livelihood: selfish children who look upon their father only as a meal ticket.

I think entry ticket, passport or key would all be ideal for the situation you describe, e.g.:

Geoffrey is our entry ticket to the control centre.

She's our passport to the secret lair.

He's our key to getting in.

One possibility is the phrase entry token. It denotes an entry pass or voucher. At the same time, token provides a suggestion of “included because we had to”.