I've heard chamomile encourages essential oil production in companion plants.

Garlic is a gardens friend, repels aphids and other bugs quite nicely.

Sweet Alyssum makes great ground cover, with shallow roots that won't compete for nutrients and most importantly provides a great home for beneficial bugs.

Legumes (beans) have done me well when companion planted with heavy nitrogen feeders, like corn. Legumes are nitrogen fixing plants, meaning they add more nitrogen to the soil then they take out. They do like to climb though, which may not be a good thing for MJ.