In that case you could probably keep it fairly informal and to begin with just put up notices in local bike shops or on Gumtree etc. Once you've got a group together it might be an idea to put something together as a club constitution so that everyone is clear on why it exists and what to expect/ is expected from them. I helped do something similar at university for a group of like minded folk that wanted to take part in expeditions. A bit more than hill walking but a lot less than going to the North Pole. We just got together in the back room of a pub once a week and shared ideas until we had the basis of a club and then started putting together events. There was no real hierarchy other than what the university instisted upon (president, secretary etc,) and those were really just nominal. Everyone had an equal say. It worked pretty well.