The most common reason for a user account to go "missing" is the lack of a role being assigned, or that user may be blocked.
The Members page only shows users who've been assigned roles, so they won't appear on this page. However, if they're still in the site, you won't be able to add them again. Instead, you'll receive a message like this:
There are two easy ways to find a missing user:
1) Use the search field to look for their email address
The search field is located in the top right hand corner of the screen, near your account name.
Then select the users tab after the screen re-loads
The click on the name to go to the user's profile, where you can click the edit tab, to add roles or make other changes.
2) Go to the "Members" menu item
Navigate to: Members
Use the "Filters" button to checkmark the button to "show blocked". This will ensure that you're seeing all active and inactive accounts within the site. Then use the search button along the top right of the table to search for the user's name.
Blocked members will show up with a red text label indicating they are blocked.
You can then edit the user by clicking the pencil icon at the end of their name. If they're blocked, you'll want to change their status to "active" (also make sure they have the necessary roles).