Within the invitation email you received, click on the large blue Activate button.

Create a unique password. Then, click the blue Save & Sign In button. Note: your password must contain at least 8 characters, at least 1 uppercase letter, at least 1 lowercase letter, at least 1 number, and at least 1 special character. Your password may not include your name or your email address. 

In the left hand navigation, click on Preference.

Click on the Connect link next to the Microsoft Office 365 option under Sign-In Accounts.

Click the blue Accept button.