One term that could be used is coaster.
It was hard to find a dictionary that listed this meaning of the word, but I managed to find one on the Wordnik page. One of Wordnik's cross-listed definition reads:
coaster (n.) A loafer.
In my experience, the word is often used when a member of a team is relying too much on others, and not contributing enough. This could be a sports team, or a team working on a project in school or the workplace. The verb form was used in a headline earlier this year, when a reporter wondered:
Dwyane Wade: Is Miami Heat Star on the Decline or Just Coasting?
As a synonym, you could probably use slacker. NOAD defines the word as:
slacker (n.) A person who avoids work or effort.
If the context makes it clear that the slacker is a member of a team, I think the word can carry the meaning you want.