I'm new to umbraco and creating a multisite installation.

The sites are for sister organisations and they want to share some content and will be managed by the same team of users, hence the one installation.

My issue is that each site has its own completely separate members. 

I know I could use member groups for each of the sites members, but for each site members will also belong to companies, so if I had a separate install for each of the sites my member groups would be companies. 

I know I'll need to role my own membership/role providers and I'm thinking my options are:

1) To have nested member groups, so I would have a "site" group which could contain child "company" groups

2) Create separate membership sections - one for each site...I think I would actually prefer this...as from an administrators point of view it would make things much clearer....is this possible?

Just wondering if anyone with more experience of Umbraco has come across a similar problem and could offer some advice?

Thanks in advance!!