In my opinion there should be some incentive to create a party and not just so u can do co-op play.
For that i would have 2 suggestions.
1. First suggestion would be to adjust the rate of crown drops a bit when u go in with a bigger party (the increase in crown drops should be roughly about 35-45 Percent per player in the party in my opinion so that solo runners still get alot more crowns out of one depth but teamplayers should have enough crown drops to keep going too in lower lvls).
2. Second suggestion would be to lower energy costs of elevators for partys with more ppl. Example 2 ppls =8 energy, 3 ppls=6 energy, 4 ppls=4 energy.
I would say that just using one of those suggestions would greatly increase the willingness of ppl to create partys in lower depth too.
After Tier 1, and really even in Tier 1, but more noticeable by Tier 2 and beyond is the fact that going through the floors with a team is much more beneficial. You die less, if you do die, you can be revived for free (no Energy used). I think there is really incentive enough to play the harder parts of this game with groups, friends, guild members etc...
Secondly why would we discourage 1 player play, shouldn't players be free to play this game how they want? Whether it be alone or in a group of 2 - 4?