this i how i join granny squares.
(normally i would crochet the last round with the same color as the joining yarn)
1// place the squares right sides together
2// start in the corner gap by putting your hook through the gaps on both squares and slip stitch 3//
4// slip stitch in the OUTER LOOPS only to the next gap
5// see, outer loops only
6// when your at the next gap, repeat step 2//
7// this is what it looks like from the wrong side, the backside of your blanket
8// ans this is how it looks on the front! when you've crocheted the last rounds of the square in grey, it will be invisible :-)
have a nice day! crochet a-way!