which apps should I use for collaboration?

I'm managing preparations for an international conference, and I need some tools that the organising committee can use to work collaboratively. For various reasons, the local options for dedicated disk space and intranet/LMS are not very satisfactory for our needs, so I'm looking into alternatives.

Basically, I need something that:

- can function as our repository for planning documents, meeting minutes etc, where it would be easy for everyone to add and edit documents
- where it is also easy to create and collaboratively expand notes that keep track of important planning information
- I'd like to be able to easily store email correspondence
- I need something that works cross platform (I work in a Windows environment, but several people also use macs)
- I need something where 'everyone' can easily get viewing and editing rights to 'everything'

At the moment, I'm thinking of a combination of DropBox and Evernote, as DropBox would work well as a document repository and Evernote for more dynamic documents and emails. However, I haven't quite figured out the best division of labour between them, and I'm also wondering whether there are others alternatives out there that I should consider. I also need to bear in mind that several members are not very computer literature, so I need to keep our system as easy to use and navigate as possible.

Any thoughts on how to best set this up?