First off I apologize in advance if this is old news, I had did a lot of looking and the consensus that I had found was it's not possible.
A couple of disclaimers about this:
1. It is not nearly as convenient as the PS3.
2. It can take a bit of time to set up multiplayer games because of how it has to be done.
3. I'm not certain what and how many games this works on. Castle Crashers does indeed work. I will be trying out if it works for Brain and for DLC later today, but I don't see why it wouldn't.
With that said here is the way to gameshare Castle Crashers between four people with one person buying it.
1. Person A has CC on their Xbox.
2. Person B recovers A's gamertag and downloads CC on their Xbox.
3. Person B signs into CC and waits until the main menu.
4. Person B then presses the center button, X to sign out, and changes to their main account.
5. Person C and D repeat steps 2 through 4.
6. Person A recovers own gamertag and signs into Castle Crashers.
7. Set up private match and invite all players.
Again this isn't very convenient and I apologize if it's old news, but if you have some friends with certain multiplayer games that you don't want to buy, this is an easy (but slow) way of sharing games over XBL.