<div>
Add a UPN suffix for the new external domain in your AD forest, then create the new accounts using this new UPN suffix. See here for steps on how to add a UPN suffix to your environment: <a href="http://support.microsoft.com/kb/243629" rel="ugc nofollow">http://support.microsoft.com/kb/243629</a><br />
<br />
As for their <b>pre-Windows 2000</b> username you can just add a prefix for the new domain to the front of their username to separate it from their existing main account if you need to use the same username.<br />
<br />
e.g. if a user already has <b>john.doe</b> as their current username, you can just assign the new account the pre-Windows 2000 username of <b>nc-john.doe</b> (nc being short for New Company - you can change this to whatever you want) so it doesn't conflict with the existing <b>john.doe</b> account.<br />
<br />
End result will be<b> john.doe@newdomain.com</b> for their <u>user logon nam</u>e and <b>domain\nc-john.doe</b> for their <u>pre-Windows 2000 username</u>.
</div>


<div>
Here's some screenshots of how I would manage logins for separate companies in a multi-tenancy setup using UPN suffixes.<br />
<a href="https://filedb.experts-exchange.com/incoming/2014/11_w48/885390/Current-Account.png" target="_blank" title="Current-Account.png (5 KB)"><img alt="Current-Account.png" class="file-block lazyload" style="max-width: 422px;" data-src="https://filedb.experts-exchange.com/incoming/2014/11_w48/885390/Current-Account.png" /></a><a href="https://filedb.experts-exchange.com/incoming/2014/11_w48/885391/New-Account.png" target="_blank" title="New-Account.png (6 KB)"><img alt="New-Account.png" class="file-block lazyload" style="max-width: 422px;" data-src="https://filedb.experts-exchange.com/incoming/2014/11_w48/885391/New-Account.png" /></a>
</div>


Current-Account.png

New-Account.png

<div>
how do you manage passwords<br />
its a hassle with 2 accounts passwords expiring etc
</div>


<div>
Comes with the territory when you have multiple accounts. There are, of course, third party tools out there which can probably help your users manage their passwords but I can't personally recommend any as I don't use them.
</div>


<div>
ok just 1 last thing I was wondering I created an srv dns record for autodiscover @ the new domain name<br />
however when I log in as the new tenant users I get hit with a security certificate<br />
the first 2 have green ticks but the <br />
last check says the name on the security certificate is invalid or does not match the name of the site which is obvious as the domain name is different<br />
is there any way around this<br />
if I click yes it works but don't want users to have to click yes every time they login
</div>


<div>
fantastic advice thank you very much<br />
helped me out greatly<br />
<br />
thanks again
</div>


<div>
Not a problem, glad I could help!
</div>


<div>
<div class="content wysiwyg-content">
im setting up a new external company email through multi tenancy<br />
this domain will have users from my main primary company and also the new &nbsp;external domain as they both will work on a project<br />
what is &nbsp;the best way to have my own users setup on the new domain<br />
I need to keep in as simple as possible<br />
I wont be able to create their new accounts using firstname.secondname as they will already have their primary accounts configured and it wont allow me to use the same sam name<br />
what is the easiest method to set this up for my main primary domain users to access their own email box and the new tenant im setting up
</div>
</div>


im setting up a new external company email through multi tenancy
this domain will have users from my main primary company and also the new  external domain as they both will work on a project
what is  the best way to have my own users setup on the new domain
I need to keep in as simple as possible
I wont be able to create their new accounts using firstname.secondname as they will already have their primary accounts configured and it wont allow me to use the same sam name
what is the easiest method to set this up for my main primary domain users to access their own email box and the new tenant im setting up

exchange mult tenancy username method

Exchange is the server side of a collaborative application product that is part of the Microsoft Server infrastructure. Exchange's major features include email, calendaring, contacts and tasks, support for mobile and web-based access to information, and support for data storage.

Exchange

Microsoft Outlook is a personal information manager from Microsoft, available as a part of the Microsoft Office suite. Although often used mainly as an email application, it also includes a calendar, task manager, contact manager, note-taker, journal, and web browser.

Outlook

Windows Server 2008 and Windows Server 2008 R2, based on the Microsoft Vista codebase, is the last 32-bit server operating system released by Microsoft. It has a number of versions, including including Foundation, Standard, Enterprise, Datacenter, Web, HPC Server, Itanium and Storage; new features included server core installation and Hyper-V.