If you're in Greenville, there's probably a kayak or outdoors shop in town. They may have a message board where you can put up a post looking for partners. Are there any clubs around? that's another good way. You should probably start off looking to boat with folks on easier water just to improve your margin of safety going with folks you don't know. Another idea is to go to a park and play spot where you can meet folks that may be willing to have you along with them after they see you on the water and can judge your skills.
As for boating alone, this is generally one of those "don't ever do it" things, but lots of us make an exception if these three conditions ALL apply: 1) we know the run really well, 2) the river is well within our skill level, and 3) we expect to see plenty of other people on the river who can help out in case of an emergency.
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse