I'm no group theorist, so I can't say for sure. I'd wager that there are some things in group theory you can do just fine without representation theory, but I'd also wager that every group theorist probably knows a lot of representation theory and has used it extensively at one or the other point in their career.
I don't think it's a subfield of group theory. Representation theory is very interdisciplinary. By nature, it involves both group theory and linear algebra, so only classifying it as either of that would already be a misnomer. Then, a lot of representation theory focuses on, say, t…