Years ago, I purchased along with legume seed, a package of "nitrogen fixing bacteria" designed for peas and beans.
Usage was easy: place a little water in a zip-lock bag, add seeds, add 1/4 teaspoon of the black bacteria, and shake for seed coverage.
Any residual in the bag would be used to inoculate any additional legume seed as only used a light dusting on the seed
I had the best crop of Sugar Snap and Oregon Edible Pods, ever, thanks to the innoculant (nitrogen fixing bacteria).
I haven't seen bacteria innoculant, though, in recent years.