Exactly what the description says.
Control points can be found all over the map. You can initiate a control point attack by holding
on your gamepad once you are close enough; you'll be prompted for this on your HUD. I don't think this is possible until you've completed the first and/or the 2nd story mission.
Each Control Point attack consists of two phases usually. First you assault the control point. Killing the last enemy will trigger the 2nd phase to defend the control point. After the defense is complete you've successfully taken over the control point and the achievement should pop.
Not sure if it was a glitchy or not but someone from my group died during the attack phase and went ahead to respawn and run back. Achievements still popped as we had no deaths during the defense phase.
You don't need a full group for this, one person will do.
I got this in a random group that just stuck around after a story mission ended.
Good luck, Agents!