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.
(After making a faux pas, doing something the group would consider uncool, etc) Come on Jason, get on the trolley.
By using the idea that if something is inside you, you are then outside of it. Commonly used for drinking buddies when the drink is placed in front of you for enthusiastic consumption.
Alternatively chicks could use it to describe a guy they fancy getting a length from
*Returning from the bar*
"Get on the outside of that Fella!"
*watching hot bloke walking by*
"heygirlfriend, wouldn't you just like to get on the outside of that!"
The expression "get on the ball" is a phrase that was commonly used in the Ottawa Valley, Canada to mean "get with it". A local derivation often used was "get on the ball or get on the bus..." meaning "get with it" or "get out of here".