Here are a few things to look for in the user management interface. Here is a screenshot with some helpful tips that shows 3 different user account states:
The first two users have both been sent emails for their accounts as you see they both have a status icon of a green check.
The third user hasn't been sent an invitation as the status is a yellow alert sign and you can see the email icon in the far right column.
The first user has fully activated their account as indicated by the green check marks under both Status (email sent/admin activated) and Verified which means the user got the email and followed the steps to complete account set up.
The second user has been sent an invitation but hasn't activated their account as indicated by a green check mark under Status but a red check mark under verified.
This may mean that their invitation is in one of these conditions:
Not delivered, blocked at the mail server
Best option to resolve have them go to the login page and select "forgot password" and follow those steps.
Sent to spam and not opened
Search spam folder and follow steps in the email or go the "forgot password" route.
Received but final activation steps not followed
Follow the steps in the email or go the "forgot password" route.
You can also resend the invitation for any of these in this state if needed. You do that by clicking on the "edit" pencil icon and clicking on "Resend Activation Email" at the bottom.
The third user in the list has the email icon on the far right side and this indicates that they haven't yet been activated by the administrator. Simply click the email icon and that will start the process to send the invitation and now their account will look like the second user until they verify their account by setting a password.