• Select "Insights" on the top bar of the home page. 
  • Then select 'Recognition' then 'Engagement'

Invite Everyone at Once

Look at the "Application Usage" box and click the blue "Invite Everyone Who Hasn't Logged In" button to remind those to login who haven't.  

Send Individual Invites 

Go to "People That Haven't Logged In" on the bottom of the page.  You can see those who haven't logged in and invite just those that you want to.