CSOs are Civil Society Organizations, also known as NGOs 
Non-governmental organizations), and in the US, non-profits.  This from 
an international list about whether individuals, or only groups, should 
be included on the steering committee to advise OECD:

"There is a large and growing literature on CSOs that questions how they 
can claim to represent various constituencies of civil society when the 
latter haven’t charged them with representing their interests and 
probably don’t even know that the various CSOs exist (i.e., 
“professional civil society” speaking for “civil society” can be 
problematic). The literature suggests that most CSOs at best represent 
the positions of their membership/employees and, even then, tend to 
represent the views of those in the organization who are the most vocal 
and/or active.

www.peoplewho.org