Some things to try:
AMC - they run a winter hiking program. I think you can find info on the general site for the AMC, as well as from both the Boston and NH chapters.
https://amcnh.org/
https://amcboston.org/
https://www.outdoors.org/
I've also run into Sierra Club groups in the Whites, though that was summer.
Hiking Buddies NH 48 - a Facebook group designed to connect folks for hikes
https://www.facebook.com/groups/956560951448114
MeetUp - I haven't looked this site specifically for hikes but it's generally used for group events.
here's a group in NH:
https://www.meetup.com/4seasonshikingclub/
You could also hire a guide to go with you, and teach you some skills. I've heard good things about Redline. They have a "winter" page under "education" on their website with info on a course.
https://redlineguiding.com/
A good place to learn is SOLO in Conway. I took their excellent Wilderness First Aid last winter, and we did some of the exercises outside. You also meet a bunch of good folks there.
https://soloschools.com/
Winter is inherently more dangerous than summer, so please learn about the hazards, the techniques, gear, skills etc. Be ready to turn around before you reach your limit; be well aware of the weather, and so on. It's also a wonderful time to be in the mountains - less bugs and sweat.
On a popular trail, on a good or mild winter day, starting in the morning on a weekend, you won't be alone. A lesser peak, mid-week, bad weather - you may be alone. So, it is possible to just meet folks on trail.
