Social gatherings where not everybody is invited

Geoff1963

at the end of our formal activity, the core group are obviously peeling off

That's the key.
Hold it at a different time, and call it something unrelated to the hobby activity.

getmore4less

how does it go, if it walks.......

that's the problem, pointless trying to pretend otherwise.

nothing wrong with sub groups just develop solutions that make it non obvious

make the plans before the meetup.
don't talk about the after party at the meetup you just turn up at the house you agreed in advance.

if appropriate have alternative after meetup events at places that can accommodate everyone, coffer shop pub....