It is slightly geared towards groups, but that doesn't mean you can't enjoy it solo. Enemies get +40% health for each player in the world, so the devs did put at least something in place to balance the game for solo players. That being said; jump in/out server gameplay isn't as much a thing in this game because of the way progression works and how the game world gets more dangerous as various bosses are defeated. Because of that, multiplayer gameplay usually takes one of two forms for this game.

1. A group of players start the game together and progress together. This gets around the issue of the game world advancing past where people are at, but players have to be patient enough to wait for everyone to be on before continuing.
   
   
2. Mercenaries: Players keep their old gear around while progressing so that they can use the old gear in other people's worlds to help them out. Jump into other worlds using old gear to make sure you don't steamroll everything. When you're done helping out, maybe they give you a little somthin' for your time.