This, and talk/type in the groups, be social and give people a reason to remember you and try to strike up friendships/join your groups in the future. This goes a long way to finding repeat groupers and private channels.If you just go into quests with the Public Group option ticked, you will often find your groups fill quickly. The exception to this is if you are unable to open Elite / Reaper and enter on a lower difficulty. You'll find that many people will not join a Norm/Hard group because they are just working on past lives and trying to get through content as quickly as possible.
I'd help if I could but at work atm so unable to log in.