Do I just constantly refresh pages and hope there will be open sign ups one day?
That or you know someone who has an invite. There's places where you can buy invites, but many private trackers will ban you for doing so. I've heard that there's ways to get into very desirable trackers by seeding your way through a few different trackers, but I'm not sure of the specifics, I'm sure someone else can elaborate on that.
Some have an interview process in place (notably redacted). Once you get in a first one (either through interview, open signup or any other way) you can start climbing the ranks, at some point you should have access to an invite section on the tracker forum where you can find invites to other trackers (not all trackers have an invite forum and they don't have access to the same invites).