For example Group 1 I can assign two tanks by name, then say I want a shaman and a hunter and another warrior in the group. If a player isn't assigned by name anywhere else, they fill into those spots.

Group 2 I can assign 4 melee 1 shaman, Group 3, 4 melee 1 shaman. etc etc.

Any spot that doesn't have any assignment can be filled by any player, but that player can be moved later to fill an assigned spot if it opens up.

Potential Options off the top of my head:

Class (Hunter, Rogue, Shaman, etc)
Melee
Ranged
Caster
Any (Allows anyone to join that slot)
Reserved (Save spot)
Empty

I would like to say by spec but there's no easy way to do that in classic. Perhaps a keyword could be whispered to the raid leader "!spec Prot" in which case we could add:

Healer
Tank
Class [Healer] (Shaman [Healer], Priest [Healer], etc)
Class [Tank] (Druid [Tank], Warrior [Tank], Paladin [Tank], etc)

All of these could be remembered so it only has to be done once unless there are spec changes.

I understand that you can't change groups with addon in combat, but I think a feature like this could be great if you wanted to set different comps between different bosses and only cared that the group makeups are good, not who is in those groups. Also forming pugs, random raids, pvp raids.

Thanks for the suggestion. Yeah this could definitely be useful but without a good, sure way to determine people's spec, it feels too clunky/complicated to implement with a raid of 40 and crowd-sourced input on role/spec.

What we really need is a widely distributed addon library that is able to more intuitively determine people's spec and communicate that among those who have the library installed. It could do that to a very high degree of confidence without user input by inspecting gear/talents. And also determine useful information about specialization such as yes, a mage is DPS but is he frost/fire, winter's chill, arcane power, aoe farm, etc...

Coding such a library would be pretty fun but i don't have an idea at the moment for a front end project on how to widely distribute it. Maybe like an in-game raid signup sheet for big guilds. Basically some raid leader would open a new signup in game, and everyone with the addon would have option to sign up, the addon would then list their name, spec, etc... with limitations on how many of each spec raid can have. The state of the signup would persist on the raid lead's computer. with potential copies distributable by the raiders. It is hard for me to test this though as my guild is small and has no use case for this.

Probably stuff like DPS vs. Tank is tricky to determine and is a bunch of extra work to implement that detection. But I imagine if you only restricted this feature to very strictly defined, easy to detect categories (such as class + "any" + maybe class combinations (mage OR warlock)) it could be implemented with a moderate amount of effort

We are a small guild with ~60 members, so we nearly know all players and which player plays which spec/role. But would be cool when it is possible to save it in the Addon (like tagging). So we can build placeholder layouts like Group 1(Tank X with random Shaman Healer).