It’s bugging out right now, probably because of the massive amount of traffic, but once things calm down you should be able to search for a magazine by full URL (https://kbin.social/m/gaming, for example) in communities, switch all search settings to all, wait for your instance to pull in the magazine and click through to the federated URL (should look like https://lemmy.world/c/gaming@kbin.social, assuming you’re on lemmy.world)

Basically, just like adding a community nobody else on your instance has added before.