Web auth
For setting up a basic user/password to allow web access you would use the "web auth" function. This is covered in the Concepts & Examples guide Volume 9 User Authentication. See page 67 and following for the sample configurations using internal and external user groups.
http://www.juniper.net/techpubs/software/screenos/screenos6.2.0/index.html
This is the sample for internal users from the manual.
CLI
1. WebAuth
set webauth server Local
set interface ethernet1 webauth-ip 10.1.1.50
set interface ethernet1 webauth
set auth-server Local timeout 30
2. User Group
set user-group auth_grp3 location local
Use the following command to add auth users to the user group you have just
created:
set user-group auth_grp3 user name_str
3. Policy
set policy top from trust to untrust any any any permit webauth user-group
auth_grp3
save
NOTE: The security device determines a local user group type by the type of members that you add to it. To make auth_grp3 an auth user group, add an auth user to the group.
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There are also options for subscription services for web filtering and internal white and black lists if that is of interest.