Well, our avg. group size is more like 4 characters maxing at 6, initially you will have one or two. So any soloplayer would have to hire help (or borrow from friends) most likely, who you then control directly. Kinda like Baldur's Gate or Jagged Alliance.
The amount of talents required alone (hacker, mage, street sam) merits a mixed group. But technically you can usually try solo...it will be much more challenging in some missions, but we don't force group play.