Method 1 Edit
You need to be on Forge to do this, and have at least two players.
To get out of High Ground, Player 2 must get into the backseat of a Mongoose. Player 1 then turns into Forge mode and picks up the Mongoose (You must be in the back seat to do this, or you will climb out). Player 1 then pushes it behind the rocks on the left (if they are facing towards the sea) and Player 2 climbs out. Then Player 2 turns into forge mode and can climb up to the edges of the map.
Note: It is easier to do this glitch if you overload the map. This glitch also works with the Warthog
Method 2 Edit
Spawn a teleporter outside of the map, and spawn a second one on the map. This way can be easier if you don't understand the first method.