Call UA, have both record locator numbers available, let them know you are the UA Gold and would like to have them put your companion in E+ on the flight(s) where you are together. It can also be helpful to know where you want them seated before you call - it does not need to be right next to you so choosing an aisle and window can (rarely nowadays) sometimes end up with an empty middle for you both.
In the event equipment changes and they reseat the companion in E, just call back ASAP to fix. Babysitting the reservation online is useful to keep an eye out for this as they don't usually send notice of equipment change. Companion can board with your group also, just hold their bp or tell GA they are with you when the non-Gold bp beeps at the gate.