resizing grouped layers

Without any specific info on your project we can't really tell you what would be the best method since it's not even clear what you mean by groups, given that AE has no such thing, but quite generally one would either parent all layers to a Null object or pre-compose everything and resize it as a whole. Anyway, for specific advice you gave to clarify like providing a screenshot of the timeline to illustrate the structure.

Mylenium