Grietjekarwietje: Het netjes afwerken van je haakwerk - This is a cool way to do an invisible join. The page is in Dutch, but can be translated by clicking an icon to the right. There are also photos which clearly show the technique.
Flat Braid Crochet Join Tutorial pattern by Margaret MacInnis. It’s a big photo-intense pages. Great idea for joining squares if you must. I really hate granny squares - you do all the squares and then spend ages joining the d----d things.