I've seen medals awarded for successfully completing a grab in ME3 multiplayer. So what is a grab and how do I do one?
If you are in cover, and an enemy is in the other side of the cover, you will see a blue fist. Hit your melee button, and you will reach over, grab the guy, pull him to your side, and then kill him. This is the grab medals are awarded for.
They are difficult to pull off because enemies will run away if they see you enter cover opposite them.