Suggested text to use when sending out details to get MFA users set up (replace text in [square brackets] as appropriate):

Hi [user name]

As part of a security update, we have had to reset your [company name] email password and have also enabled an additional security feature called "Multi-factor Authentication" (or "MFA"). This adds an extra verification step which is attached to a physical device you own in order to prevent remote third parties from accessing your mail even if they manage to compromise the password.

To get set up with this correctly, please access https://outlook.office.com/owa and enter your [company name] login address followed by your new password (below). The first time you do this, you will be required to set up MFA using a phone or other smart device - once configured, you will need to have your phone/device handy in order to authenticate your [company name] logins going forwards. 

Microsoft (the MFA provider in this case) has provided more detailed documentation on how to set up your MFA device(s) here. As noted there, the recommendation is that you use the Microsoft Authenticator app on a smartphone, but you can also use alternative methods if that is not possible for you. Your login credentials are [email login] and [email password].