Advise them to check their Inbox and Spam or Junk mail.
If they still do not see the email, ask them to add email@example.com to their email Contacts and attempt to resend the credentials from the client record.
Then go to the Staff or Client tab, find the user in the list, click on their record and click the button "Resend notification to establish credentials".
NOTE: If there is not a link to "Resend notification to establish credentials", then they have already previously completed the process. If they need their username, you will need to provide that to them. If they don't know their password, they need to go to your Office Portal and use the Forgot Login option.