A bocking group bocks!
Well, firstly all the clubs are set up for fun, as more people in the same areas get together, some have just decided to collectively give themselves a name.
They are mostly free and usually if there's a gym meet that you attend you will be asked to pay towards the fee for the session which is usually just a few quid.
Many members are part of a few groups and just try to get themselves to as many meets as they can, but usually they try to commit themselves to one group which is more local to them, But its entirely up to you.
Also as Bbmthbloke often says just turn up to any meet you fancy, no one will turn you away