Adding a password to a set
To add a password to your set choose "Visible to: people with a password."
You can choose this when creating a set, or access this anytime by clicking "Edit" on the upper right of the set page.
Using a password protected set
Password protected sets don't show up in search. The best way to give others access is by sharing a link to your profile page (in this case, it is quizlet.com/quizletsupport).