guix:
Ok thank you guys, I hoped there was a more clean solution... A terminal connector and it's short bar will do it. Anyway, it will be hidden in a box
Another possibility (though a bit more dangerous) would be to use a bus bar; these are sold typically a "big box" home centers (Lowe's, Home Depot) in the electrical aisle, near the circuit breaker panels/boxes. They are basically a piece of metal with a bunch of holes, and screws which are perpendicular to the holes; you insert your wires into the holes (typically solid core - this is meant for home wiring in a circuit breaker), and tighten the screws to secure the wires. They are meant for tying all the independent neutrals and/or grounds together:
They also come in other styles, but this is the kind fairly common here in the USA for residential usage. You could instead build your own - get a piece of solid square aluminum rod, then drill and tap holes for your wires - use spade or ring terminals on the ends of the wires.