After players select an Allegiance (and an Envoy, if desired), players choose which Commander(s) they wish to lead their Company. Commanders chosen must belong to the Company's Allegiance.
The envoy cards are what it means by selecting an envoy, the guild/court card specify you can take a commander from those factions, the other nations don't so the only way to benefit from their envoy cards is a commander that is dual nation.
I'd love to take the dragon titan commander with my kings empire but RAW you can't