I want a one or two word title for a character who is included in a group only because he is their "ticket" to get where they want to go. "Ticket" is what I'm thinking now but would love another option. Basically, where the other two are trying to go they can't get through without his permission.
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”.
An official document or an escort assuring unmolested passage, as through enemy territory.