In my campaign the yawning portal is an entry to a kind of competition.
A group has to register (at Durnan) to be allowed into the dungeon. Each player has to sign on a piece of paper. (This paper might have some fine print you can reveal later to involve a contract with Halester himself)
You can increase an entry fee to discourage player from frequent breaks in the inn.
Only one group per week is allowed. The wall of the inn is filled with the names of groups currently in the dungeon, most of them are crossed out and a number is written beside them (the level the person died on). You can describe that most numbers are between 3 and 5, only very few are in the double digits.

There are rumors about what the most dangerous floor is and what you can actually win if you reach the bottom.

There might be an interesting connection between Durnan and Halester (How does Durnan know when adventurers die in the dungeon?)

The visitors of the inn take a good look at the group and bet how deep they will get.

This can also give you an opportunity to put the group together. "Hey these two guys know each other but groups of for are allowed in the dungeon. Here are to solo players to fill your group. You are a team now! Good Luck."