Have you checked meetup.com? In my metro area they have several very active hiking and backpacking groups, which I've heard positive things about from other local hikers. They range from easy to quite challenging in their focus.

I don't mind solo hiking but will probably try participating in one or two of these hikes to get a feel for the folks and the organizing.

We also have very active traditional groups like the Sierra Club, etc.

Cheers,
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--Rick