I started a co-op and invited some of my friends, no response. Do you have to be friends to invite someone to join your co-op? Can you only become friends with people who are 'suggested' or you know their code?
Are your friends still active? Yes you have to be in-game friends to invite players to a co-op. You can add friends either through suggestions, Facebook, or in-game from finding a town or using a players code.
I'd suggest joining an active co-op and golden league one as well if you want to be competitive. There's far too many players creating co-ops which is a problem because the player base isn't enough to compensate for it. No matter how much you advertise a co-op it won't get new players if there aren't any to begin with.