Add people who have used contribution page to a group (or tag them)
To my knowledge - If you have set up an unique financial type for the contribution page it would be easy or if you have an unqiue label in the contribution source that might help too 

Select all the contact using advance search - filter all the contact based on financial type id or using contribution source or date based on the contribution - in the contribution section 

once you got the list - add to a specific group manually - done

But you need to do this each time manually - may be wait for someone to give you more precise answer

thanks !!!