Windows Live Domains is an easy way for you to get free "Hotmail" type accounts for a domain you own. You simply visit domains.live.com and follow the instructions. This will eventually open up an Admin Centre that you can use to manage accounts, set-up co-branding, and generate a sign-up module. You authenticate as an administrator for the domain via an existing live id account, and by making a change to DNS for the domain you want to manage (thereby proving you own or manage it)
Since this authentication is tied to an existing live id, this seemed to imply that only one administrator account could be created. However the help file offers another solution:
You can have multiple administrator accounts for your custom domain. Each administrator must register the domain on the Admin Center website and enter the MX record (An MX record specifies which server in your domain should receive incoming e-mail.) that's associated with that administrator at your registrar's (A company that is accredited to sell Internet domain names.) or DNS (The DNS (Domain Name System) helps your computer find other computers and websites on the Internet. A domain may provide a website (such as www.wingtiptoys.com) and e-mail (such as me@wingtiptoys.com), among other services.) provider's website.
Seems like a rather strange way of allowing multiple admins, and you will be at the mercy of your DNS provider allowing multiple MX records for your domain (www.zoneedit.com allows a maximum of 5 for example), but it does work.
UPDATE: Turns out ZoneEdit does support more than 5 MX entries - but only 5 priorities, which is fine for this.
Alternatively you do have the Api which you can use to manage your domain accounts, so if the MX entries is too limiting that would have to be the route to take.
Cheers
Ian