To be "with the group" or to "be cool" as determined by your peers. The "trolley" would be something one would want to be on.
One may also want to make it a point of saying that someone is not
on the trolley, instead of just saying that they should get on it. This implies that the person is so uncool that they cannot even try and be cool.
Another popular use is pointing out that a particular member of your friends circle is
on the trolley in an attempt to rub it
in the face of the person who isn't.
This phrase is usually used in a ribbing manner towards friends.