Come/go along for the ride
Definition 1 of 2
Pro Tip 1/3Passivity
Implies a person is not actively contributing but just present. 
She's not helping with our project, just coming along for the ride.
Pro Tip 2/3Involuntary Participation
Sometimes used when someone is involved without a clear choice. 
I ended up going along for the ride during their shopping spree.
Pro Tip 3/3Minimal Impact
Indicates someone's presence won't significantly affect the outcome. 
He's coming along for the ride, but the decision has already been made.