Now, some clubs/ranges hold shoots that are "IDPA-similar," but that are not held under the auspices of the IDPA. So you'll want to check what you're shooting. If the club is shooting unsanctioned, ad hoc matches, no biggie.
But if you shoot IDPA matches, even if the club is lenient, IMHO you should join the IDPA. It's the right thing to do; it keeps the club compliant with IDPA rules; and it promotes the shooting sports.