A meeting invitation notification will be received as an email. 

To respond to a Meeting from the Invitation:

1. Open the invation, or email.
2. Look in the ribbon the choices will be Accept, Tentative, and Decline meeting under respond group.
3. Respond by clicking on either choice

 To Respond to  a Meeting from the Reading Pane: 

1. Select the invite.
2. In the Reading Pane, you will be able to select Accept, Tentative, or Decline.
3. Respond by clicking on either choice