- Select "Insights" on the top bar of the home page.
- On the left under "Dashboards" select "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.