Can you not assign group 1 to a different unit to ungroup everyone else that was currently in group 1?

I've just checked the manual (I originally bought Earth 2150 when it first came out, so I have a paper manual) and it mentions creating groups but not how to ungroup them.
I'm going to agree with Ralackk and say that you just need to re-assign the group number to a new selection of units. It does what you need it to anyway, cause if you don't want to use a group you've set up, just don't press it's number. If you want a different group of units against a group number you re-assign it by creating it again.
If I remember correctly, Earth 2150 also has platoons which you can create. The difference with these is it names them (e.g. alpha, beta, etc) and you get different functions for them. You can disband a platoon, so maybe those would be of use?

Ah well. I play Earth on my netbook, so I sometimes end up pressing the wrong group and doing silly mistakes. I think I'll just stick to creating platoons instead.