You can reset a client's password in their profile. Below are the instructions. Their username is the email address you have listed in their profile.
1. Click on clients on the sidebar
2. Click on contact list
3. Search for their profile under contact using their name.
4. Click on their name to open the profile page
5. Under email you will see "Send reset password link to user" click on the box to check it and hit Submit at the bottom of the page.
6. Once they get the link and reset their password, they will be able to log in as a client.