IANAM, but I can kinda second this from personal experience. The Masons had a display at the State Fair many years ago (I was in my 20s), and one of the guys manning the booth said “Why don’t you go to your local lodge and ask. We need new blood.”, but that, and handing me a brochure, was all he did in terms of recruitment. If it was by invitation, he presumably would have gotten my name and phone number/address, or something.
Also, one of my uncles is a Mason. I noticed his pin at a wedding or funeral or something, and commented on it. He said essentially the same thing. I wasn’t interested enough to bother then, and now, in the internet age when it’s trivial to learn about it online, I’ve seen enough to not really be interested at all.
If they were into issuing invitations, I expect that my uncle would have arranged for it to be done, after my asking him about it, and I might have joined, then. He didn’t, so I assume, as AT said, it’s not “by invitation”, like a college fraternity (I’m a member of one of those, so I do know how that’s done), but rather a case of “If you’re interested, come and ask.”