There are a few misfits who float a thread on here called "Thursday Nighters". If you're Llanberis area and don't mind the drive they're often getting their snouts dirty Gwydyr forest or Ffestiniog way. I popped out with a few of the usual suspects to Aberllyn Mine and they were all good blokes.
The chaps at Parys Mountain are a bunch of muddy bludclats and were very hospitable when taking yours truly for a rummage in their holes. The trip they offer needs no specialist gear, but do supply lid n lamp if necessary. Tip them 10 squid towards upkeep + insurance and you won't regret it.