Going down from Colonel in rank, you’d have Lt Col, Major, Captain, First Lieutenant, and Second Lieutenant. I’d imagine that if a Colonel is discussing strategies or plans, it would probably go no lower than Major. The major would then be responsible for disseminating information and planning the usage of those subordinate to him.

Someone with more knowledge of the army might be able to tell you better…I was a swab jockey.

Are they at war, or is this during peacetime?

“Discussing some military matters within the regiment” is deliberately vague, I suppose, because nearly all of those words are vague.

In the first place, though, the Army (presumed army, presumed USA) organization no longer includes regiments. The modern equivalent is the brigade, consisting of approximately 3000 – 5000 men (I use ‘men’ in the same historical way that you use ‘regiment’), which will consist of ‘a few’ brigades of approximately 1000 men each, and brigades are generally commanded by a colonel, so we can start there.

Depending on the ‘military matters under discussion’, the colonel could be giving an address to the entire brigade, or to any group he cares to form of its officers and NCOs, but this is hardly ‘discussion’, so probably not what you want.

If he’s formulating combat strategy in furtherance of his received orders from Division, then he will probably want to consult with just his senior staff officers, at least until the strategy has been determined and is ready for dissemination to the battalions and platoons. On the other hand, if just one of his platoons is involved in an operation of particular interest to him, then he may decide to meet with that platoon leader (with or without the platoon’s junior officers and non-coms) to impart special instructions or emphasis on the operation.

This might be a good place to start your research.

A regiment can be made up of between 2 and 6 battalions. Each battalion can have a full bird colonel in charge. Each battalion has at least 3 or 4 companies. Each company is lead by majors, captains and lieutenants.

You should also look up Staff Officers for the unit type and time period you’re writing about.

That is, large enough units will have departments headed by officers who focus on the types of activity those departments are for, which have staff officers who may be lower ranks, but they may be people that higher-ranking officers talk to regularly.