Is it an actual title of address, like someone would be called, "Co-Founder Jones" in the same sense that someone would be called "Mayor Jones"?
If not, if it's simply the name of a job or role, like "engineer" or "accountant", then it is not capitalized. You write, "Bob is one of our co-founders," not "Bob is one of our Co-Founders".
If it really is a title of address, then it should be capitalized when and only when used as a title before the person's name. Thus, "The meeting was chaired by Co-Founder Jones", but "The meeting was chaired by one of our co-founders". (Well, it would also be capitalized when used as the first word in a sentence, etc.)
If it is capitalized, whether you should write "Co-Founder" or "Co-founder" ... my gut instict is "Co-Founder", but I don't know of any authoritative source to back that up. There's something of an analogy that when you have two-word titles, both words are capitalized, like we write "Vice President Jones", not "Vice president Jones". I think a hyphenated title would follow similar logic, but as I say, I don't have any independent confirmation on that.