Anybody, even non-owners, can be a member of an EC if they are legitimately nominated by an owner and elected by the other owners (if an election is required). However, they can't be nominated by a co-owner who is standing for election themselves or, indeed, nominate themselves unless their name is first on the strata roll.
Someone who owns a quarter share in a flat may only be an investor and they may be on the EC to look after the interests of the other co-owners. It's unusual but absolutely legit.