Social Network Tools

I'm hoping you can help a reader with thoughts and likely help others of us …

A medium size nonprofit and lobbying organization is working on a designing and creating a social / work network for experts, students, volunteers who are willing to help through campaigning, lobby, sharing knowledge, working on solutions (writing documents).  They currently don't have any kind of solution.

Via the network people will be able to communicate with employees, organize activities, collaborate on documents, communicate with each other, etc.

They are considering a variety of solutions but are willing to balance functionality vs. cost.  In other words, they could live with a lesser solution if they can save a lot of money.

• Document sharing and collaborative editing
• Custom branding / image
• Ability to create groups and control access to those groups
• Shared calendars
• Invite other people into the network
• Moderation of content
• Exports (users, content)

Some questions:

1. What do you call this kind of solution?
2. Given this high level description, what would some initial questions/criteria that might differentiate what solutions to consider?
3. What do you think some of the safer choices would be in terms of solutions, i.e., who will have the biggest market share in the future?
4. What tools might be on your short list for them to consider?