1. For sites using the Amherst default theme: in the Administration block on the left, in the section Course administration, click on the link Users, and then on the link Enrollment methods and finally on Guest Access. 
2. On the page Guest access, in the menu Allow guest access, select either Yes or No.
3. Click on the button Save changes.

By default, courses are built with guest access turned off.

If you set guest access to Yes you can then attach a password. Only guests with the password will be allowed access.

Note that guests will have access to resources, but not to activities, E-Reserves or Video Reserves.