@vielbein if you see their posts you can click on their username and then click on the follow button in their profile.
I once had a situation where I couldn’t find someone through the the search despite knowing their account. In that case you can paste their full account url(https://example.org/@username) and then it will take you to the profile and keep you signed in on your instance so you can hit follow.
There are many ways probably but that’s just how I do it.