Yes, joining is just starting to knit, so that's fine. Garter stitch in the round leaves a jog because you're actually knitting a spiral, so the end of the round is one stitch higher than the beginning. What you can do is slip the first st of the round, do a purl round, working that slipped st as a purl when you come back to the end of the round. Then slip the next st (the new beginning of the round) and knit around, knitting the slipped st. Repeat.
BTW, if you knit every stitch in the round, it makes stockinette, not garter. I can't see a pattern start with ribbing then go to garter stitch, which is alternating purl and knit row in the round. If you're supposed to be doing stockinette, don't worry about the sts not lining up.