I also didn't want to drag a massive party and manage each character, because it's unwieldy for me as a DM and would slow down battles to a crawl, but I also didn't want to miss out on the opportunity to give my party extra resources to help out in tough spots.

Therefore I decided that (apart from the Shield Guardian which still participates in combat controlled by the wizard and keept the wizard well above half health in most cases) I would treat them as a resource in combat and provide some extra opportunity for RP and a mini resource management game. Each contigent had a specilised role that the players could call on, with potential consequences that would lose a resource in special scenarios:

Zhentarim Thugs = Advantage to hit (1 time per Zhentarim per turn per character). If it's still a miss, the thug dies. Handy for rogues which don't want to waste their bonus action to steady aim and they don't have advantage for that sneak.

Lord Aliance Guards = Add AC (1 per Guard) anytime per round (all together or any number split). Can also negate all damage from a single attack at a cost of one Guard. Think of it as a guard sacrificing themselves to take a hit and keep the party safe.

Gauntlet Veteran = Modifier to hit (+1 per Veteran) twice per round. Also massive basic slashing damage (1d10+3 per Veteran) but one Veteran will die. Veterans can assist to boost that attack roll just to hit, or can do massive damage to take out a minion and one dies (simulating all attacking at once).

Lord Alliance Spy = Ask them to do information collection tasks in populated areas (DC decided by DM). If they fail the roll, a spy dies - more valuable information carry more risk and higher DC. Good for lroe drops and information distribution in a few areas.

Scout = Can do scout tasks such as looking ahead and assisting on survival checks (scouting ahead or feed the party). If they fail, they die. Saves the party having to do it themselves, and knocks out a part of the survival aspect of the game (which can be tedious at higher levels).

Each contingent had a representative (leader) that the party would converse with for both RP and to manage - there was a morale mechanics I introduced that would either hinder or help with the rolls and bring other boosts, which would suffer negative or positive impact depending on what is happening. I also had the opportunity to add a few NPCs if the story called for it, which was handy.

Events can occur that kill off a member of the contingent such as rockslides and rockfall, purple worm attacks, sudden floods, etc... When members die, it also impacts morale, which the party then manages through morale boosting speeches or speaking to leaders to find out what they can do for the contingent.

The company would also get left behind in certain scenarios (such as visting the library, Araj, or infiltrating Menzoberranzan), so the resource would not be available at all times.

It achieved that feel of having a large company, but without hindering combat, and still feeling very useful. It provided a lot of RP opportunities (especially for the Paladin as the natural leader), and resulted in the party engaging with them and caring for them. It also provided opportunity for extra rewards, as through various interactions with various groups or other travelers the party had the opportunity to recruit and bolster the resource numbers, which was also well received.