Oh dangit - you're right. It seems not being able to follow users is a feature, not a bug in Lemmy, with the advice of the developer being to use Mastodon or kbin instead if you wish to do so. There's a new issue open about it though, so hopefully they'll give in to popular demand.
Sorry for my misinformation! Since I don't have an account over there I just assumed the follow button would appear once signed in. :/