I would love to see such feature in the game. Clan members constantly joining armies are getting quite hard to level up in some skills, and they get captured pretty often too.
Also, it would be easier to defend own fiefs if you could let companions do this, but it is impossible, because they are always joining some army. This makes picking companion party ai, like "Aggresive", "Neutral" or "Deffensive", practically useless.
Maybe Companions can only join an Army with the express consent of the Clan leader (player).
If clan leader allows companions to join, clan will earn some influence and if clan leader declines, then there should be a small influence penalty.
Also, companion independent parties should be a little more useful and have some more specific roles.
As it stands now (pre-1.7.0 as I have not tried Companion parties yet), they just wander around seemingly aimlessly and not sure how useful they are. What's missing are more specific tasks, such as for instance: security duties (going after looters/brigands), commerce raiding (attacking enemy Caravans when at war), war patrols (attacking enemy parties)...etc. Maybe this is already happening, but I have not seen it much or think it being useful at this time.
Also, in addition to the more specific tasks, we need more specific areas of operations. For instance:
Patrol Clan's territory, Companions would go and intercept looters/brigands that are active in one's own territory.
Border Patrols during war, in which Companions would intercept enemy parties entering Clan's land or Kingdom...etc.
Border Raids, in which Companions would cross into enemy territory and raid villages/intercept Caravans.
But, if nothing can be added to companions we should have the added ability to play the companion during a battle. If one of my companions is attacking or being attacked, I get a notification that would ask if I want to take control of the battle as the Companion. This could be an option that players can select if they want to take charge or just give the AI exclusive control of companions.
I guess I digressed from 1.7.0 feedback to suggestions/wishes. I will post this in the appropriate forum also.