Existing outlet, metal junction boxes, new circuits, and ground
You indicate that the &quot;A&quot; circuit just passes through that existing box at the top left and that you're adding a new box right next to it.
Assuming there's enough cable in that box, extend it to the new box and splice to your new cable run to the new box there. Do not connect any circuit A wiring in that box except to pass it through to a new piece of cable that carries on to the &quot;A&quot; box.
A better alternative, especially since you're already planning on installing a new box right there, is to use that existing box as the junction between the existing wiring and the new wiring and put a blank plate over it. Use the new box as the outlet box for the new circuit. This way there is no confusion whatsoever about how to wire and (for the future maintainer, maybe future you) there's no confusion about why wires are just passing through and, more importantly, no danger from having 2 circuits in one box and needing to remember to turn them both off for safe working in the box.
Also, I believe there's something code related about having a single outlet on a circuit (your A circuit), but I don't recall if it's a 15 or 20a breaker that has issues, so I'll leave that to the licensed electricians.