I love doing things with my pup, but it can be hard to join groups as not everyone likes or can be around pups and not all locations allow pups (ex: National Parks). I have found some success on Meetup on finding pup friendly adventures and hikes. I would check that out and also check with your local pet stores and/or groomers to see if they're familiar with any pup friendly groups.
I find that even with groups that welcome dogs, too many dogs is disruptive to a great hike experience. I ususally bring my dog along on local hikes by myself or with one other friend and dog. AND, don't forget that your dog disturbs the wildlife more than you do. So, keep 'em on the trail and don't let them chase wildlife. Also be aware that there are people who are really afraid of dogs so keep 'em close to you.
It is great to have a companion out there!