Frontify does not offer password protection. Instead, your Style Guide or Project is only available to the users you invite to View or Edit the content. Once a user is invited to a Style Guide or a Project, they need to create their user account with email and password to access it. Then by default, they see all the content in the Style Guide or Project. Only the invited users can see the content. You can see how to invite the users here.
If you want specific files to be password protected, you can set a password protection before you upload them to Frontify.