How do I get retrieve my reinforcement troops that I sent to help someone?

Simply go to the battlefield (click on the binocular icon on the lower left most part of your screen when inside your command center). Once you get to the Battlefield UI, there are icons on the left part of the screen -- depicting the groups that you sent out to gather resources or probably attack bosses or player bases. If you tasked them to gather resources, you can simply click the "Go Home" button to retrieve them. You'll have to wait for them to travel back of course before they actually get back to your HQ. During the times you have tasked them to go battle otherwise, you'll need a specific item to call them back (March Recall).