Curious about Collaborating with Tasks and Documents

[asedesigns]

I am looking to install ZIMBRA Open-source edition on my VPS server to collaborate and work on a few documents and tasks within our team and group of people. My main concern is does the team have to have ALL their own email account at my domain where i set zimbra up?

For example say i have user1@mydomain.com user2@mydomain.com user3@gmail.com user4@yahoo.com

User 1 is me, so would i be able to only share with user2 ? and user3 and 4 wouldnt be able to work with us since they are on a different domain email account?

Also how is the login process work with zimbra is it like cpanel type of thing
mydomain..com/cpanel then you go > to zimbra, or can it be like mdomain..com/zimbra for ease of remembering for our whole team?

final question is can i route multiple domains on 1 zimbra account such as mydomain2.com and mydomain3.com without installing zimbra 3 or 4 times EVEN IF those domains are on a different server? is that possible?

Thanks for your help and sorry for the long post...