Portals will appear during the progress of the match.
Blue Portals will let you escape the dungeon with your loot.
Red Portals will take you down to the next floor which has better loot but much higher difficulty.
You can sometimes hear the portal stones rise up out of the ground. There seems to be a general area of the map where you seem them most. Keep following the circle. The pinpoint at the very end IF you have to go that far had several red and blue. Enough for two full teams each about. Once you portal out for the first time you will hunt the blue ones after.