Just put a brush in your pit, apply the texture: common/trigger on it.
Select the trigger brush and click RMB. Make it a trigger_teleport.
Place a target_position anywhere in the map you want to spawn from the teleporter-pit. Select your target_position and press N to check the direction you will spawn out from the teleporter.
Now select FIRST your trigger_teleport brush and next the target_position. Once you selected both just press ctrl&k to link your 2 brushes, a blue line should be drawn between if you did it right.
Here you go with your teleporter pit.
