From Dark and Darker Wiki
There are two different types of portals in the game, one allowing you to escape and the takes you down one level deeper.
How to Use
Locate the portal and initiate its activation by pressing the F key. Once activated, a blue/red circle will materialize, and the transfer process will commence as soon as you step into it. It's crucial to exercise caution and be mindful of your surroundings, as any disruptions or enemy attacks while entering the portal could result in a failed escape. Openning any portal gives 2 Experience
Escape Portal
There are also Static Escape Portals that will appear in the same place, allowing a single person out of the dungeon.
Down Portal
Down Portals will appear in a random place in the map in a set of three, allowing 3 players to descend together, each of these portals also only allows one player to escape at a time. "When you use the dungeon portal to go down to the lower floors, you will face stronger enemies than the ones found on the normal dungeons."
There are also Static Down Portals that will appear in the same place, allowing infinite people to descend.
Down portals only spawns in Crypts, teleporting the player to Inferno
Portal Spawn Times
You can check the spawn times of portals for individual dungeons: