To send an account invitation to the email associated with your primary account login, follow these steps.
Log in to your secondary MailChimp account.
Click your profile name and choose Account.
Profile menu with cursor on Account.
Click the Settings drop-down menu and choose Users.
Settings drop-down menu with cursor on User details.
Click Invite A User.
On the Invite a user page, input the email address associated with your primary account
login and choose Admin.
Cursor clicking Admin radio button.
Click Send Invite.
Cursor clicking Send Invite.
We'll send an email to the address you provided asking you to join the account. As soon as you receive the email, you're ready to link your account logins.
Link Your Logins
To link your primary account login to your secondary account, follow these steps.
In your email client, open the account invitation.
Click Join This Account.
Cursor clicking Join This Account button.
On the Login screen, click Use your login to join this account.
Cursor clicking link to use existing login.
Input your primary account's username and password and click Log In & Join.
Cursor clicking button to log in.
Now your account logins are linked. The next time you log in to MailChimp, we'll ask you which account you want to access.
After you log in to either account, you'll be able to switch between them in the Profile menu. Click your profile name, choose Switch Account, and then click the name of the account you want to access.
If you have a data field (Merge field) in your current list that can be used to identify the people who want to be in the beta you can just segment the list and send it to those people.
Also you cant have duplication of emails as Mailchimp checks and updates the existing user.
Hope this helps if not then I can explain some more once at work (and I can then see mailchimp)
It appears that method 3 is a valid solution if you have some programming skills.
I contacted the MailChimp and Mandrill support and they both confirmed that it's possible to send newsletters with Mandrill. MailChimp even allows you to use pre-defined templates for your Mandrill newsletter.
The solution is to write a script (in a language like Ruby or JavaScript with Node.js) that will send emails to a list of addresses (it's possible to export your entire list as a CSV file in Mailchimp) in batches of 100 or so with a small time interval between each batch.
To handle unsubscriptions, you can use Mandrill's UNSUB merge tag together with the Webhooks feature. You can set up a web service that will receive POST requests from Mandrill whenever an unsubscription happens (e.g. you can set it up to POST to http://mysite.com/unsubscribe). Your web service will then have to use the Mailchimp API to unsubscribe the address communicated by Mandrill from your list.
Although this solution can be laborious or hard to do if you don't know where to start, it can save you hundreds of dollars and can be a valid way if you have the skills required.
Best Answer
Advice here (with images) includes:
To send an account invitation to the email associated with your primary account login, follow these steps.
We'll send an email to the address you provided asking you to join the account. As soon as you receive the email, you're ready to link your account logins. Link Your Logins
To link your primary account login to your secondary account, follow these steps.
Now your account logins are linked. The next time you log in to MailChimp, we'll ask you which account you want to access.
After you log in to either account, you'll be able to switch between them in the Profile menu. Click your profile name, choose Switch Account, and then click the name of the account you want to access.