There are many groups studying individual non-linear and complex systems.
What I have not been able to find is anyone who is trying to understand the nature of non-linerarity and why it leads to complex system behaviour. Part of the problem lies in teh definition of complex systems.
With regrd to the definition of complex systems there are many whihc are vague and IMHO are incomplete. Here's a reasonable one.
"'Complex Systems' is the new approach to science studying how relationships between parts give rise to the collective behaviors of a system ..." Introducing Complex Systems, Y. Bar-Yam, New England Complex Systems Institute, Cambridge, MA, USA
In my view, the most impoortant part is the "give rise to". This is the opposite of reductionist science and requires the use of different analytic approaches as well as conditions for determining validity of conclusions.
Areas of study I would have though deserve attention are: