From the little explanation it sounds like it is kinda like making mini Teams inside of your already set up teams.
This is correct. For example i have seperate action target groups for "TANK", "HEAL" and "DPS". That way i can send appropriate action to specific group and only to those who actually need it. This makes switching roles within group easy.

I usually run with tank, healer, 3x dps, but with shamans i also like to run 4x dps when dedicated healer is not needed. I can easily swap my healer to dps with a push of a button, change action target group HEAL -> DPS and Voilà! my healer is now dps!

I also use Virtual Keymaps which makes swapping classes and roles even more easier.


Also if I make a keymap under this group can I put toons from seperate teams into it
Yes you can. I have 13 characters split between 3 main groups.


For example: Lets say i have 15 toons split into 3 teams, and each of these teams has 2 druids in it. If I make an ATG group for a druid spell for them can I put all the druids in there and when I am running whichever team, the 2 druid in that group will have access to that keymap.
Of course you can make ATG for each class if you wish.

I would also advise you to look into Virtual Keymaps. For example you can send Virtual-DPS-Dummy-key to your toons and each toon can assign their own DPS-keymap to that dummy-key. This way you don't need to worry if you're sending correct action to different toons.