There are a few sections restricted to solo only, but it's not the default, the matchmaking is pretty quick for a random group and there's a variety of people always looking to form groups for different tasks. One word of warning, people move fast, until you get parkour down, you might just end up running from the start to end of a level if you join groups, they'll have completed the objectives and be waiting for you to extract.

Clans exist, and each have their own space station called a dojo that's customized by them (cost is based on size of the clan, as a solo I was able to build up and level a clan on my own).