Linux

Not me over here thinking you meant Lego Users Group! XD

My recommendation is meetup and a website for advertising purposes. Meetup is frustrating, yes, but at the same time it's where I have found almost all the linux and tech groups near me.

Oops! I ended up writing a wall of text...

I would suggest using methods other than platforms to find events, as there are lots of people that don't use them. In fact, where I live, I've never seen people using that kind of platforms. I guess this can be different for you, but I think you should at least consider other methods of spreading the word, to guarantee that some people don't miss the event just because they never heard of it. Here are some.

1. Printing posters and placing them in visible places. It might sound a little old-fashioned, but it gets the job done, especially for local stuff. When doing posters, you have two things to worry about. The first one is what you will write in the posters. You of course have to clearly state what you want to do ("Let's start a Linux User Group in our town!"), but you also have got to explain what you were thinking the group could/should do (for example "Let's organize monthly instalation parties", or "Meetings every two weeks", or whatever you thought about doing). That doesn't mean that the LUG must do exactly everything you write in the poster. It's just to give an idea of whether the LUG's activities will be of any interest to who's thinking of applying to it. Put some contact information on the poster. It would be interesting if you decided to organize a gathering with people that would like to be founding members of the LUG, just to figure things out. If you decide to do that, don't forget to put the meeting's date and place in the poster. The poster should be eye-catching and have all information clearly visible, while highlighting the most important information. If graphic design isn't your passion, ask someone you know for help. The second thing you should worry about when doing posters is where to place them. As far as I know, only nerdy people would join an LUG. As such, you should put most posters in places where lots of nerds gather together, such as places where people gather to play board games and tabletop RPGs, electronics stores, ... Even then, you could try putting them in places that are more general and have more visibility. For example, I've seen some supermarkets with boards for event announcements. Just don't forget to ask for permission to put the posters whenever you want to do it in a place that isn't the street. As long you explain what you're promoting and say it's non-profit, most places will allow it.
2. Advertising on forums/social media groups related to either Linux or to the place you live in. Much of the advice I gave about posters applies. In places where you know you won't really be checking constantly for new messages, you might want to redirect people to your contact info (say something like "If you want to contact me, instead of sending me a message through this platform, please use my contact info"). Also make sure to follow the rules of the places, and contact the admins if you have questions. I don't have much experience with this approach, so I can't give much advice.
3. Asking Linux users you know to promote it. Word of mouth is always useful ;)

If you could give Mobilizon a try, I'd be interested in hearing your experience.

Traditionally they've been email lists. Easy to set up and everyone has an email address.

My LUG uses discord :/ but we are working on a matrix bridge.

Bridges IMHO seem like the way to go.

Start with a email list and then try to get together in a pub or somthing like this on a regular basis.

If the LUG gains ground begin to look to some place where you can get together and eventually keep some hardware.
If possible, once the LUG has some regulars, look into joning some events, maybe as guest at a booth