Hi Janine,
Is it true that your plan is to have two subsites, under the Main Subite? That there will be some members who will have access to both subsites, but not all?
From what I have learned about permissions and groups in OpenACS, you will need to add those users who need to read both subsites, to the groups for those subsites.
The original idea of subsites, afaict was to have some content that could appear in two different subsites, and users would be restricted to just the subsite they were a member of. In this case, the users only are in one subsite, but teh shared content (somehow) shows up in both places. Most likely this was intended to be accomplished by mounting a package in both subsites.
I don't know of anyone using OpenACS this way, so we need to take what exists, and mold it to our needs, I guess you need some way to setup a user so it will be amember of two different groups easily.