And OP wouldn't have needed to explain that they are just a friendship group if she hadn't clearly and unambiguously stated in her opening sentence "I am part of an NCT group...". If it's not an NCT group why confuse matters by ever mentioning the NCT?
The NCT part's actually important. I know a lot of posters are comparing this to a husband randomly turning up to see a group of his wife's friends lots of comments like yours about married couples not being interchangeable and, yes, that would be odd.
But NCT courses are aimed at both parents, and with the inbuilt expectation that people will continue meeting up after the course is over. Socialising with other parents is a huge part of NCT -- maybe even the main part, since we all have access to medical advice and could find the info they give elsewhere. It's predominantly used by first-time parents who don't know many other people with kids. (Isn't that why NCT gets shit on MN, sometimes? It's expensive, middle-class, and you're basically paying for friends.)
As someone's already said, the course leader takes everyone's email addresses and shares them, and usually appoints one person 'organiser', with the assumption that the group will continue meeting. It's not a sign of enduring friendship that an NCT group is still meeting 4 months post-birth. It would be unusual if they weren't meeting up. It takes a while to work out whether or not you actually have anything in common with this people, other than being parents, but at the early stages you need each other for a support network, and that's why people are saying it's unfair to excluse one SAHP because he's a guy.