You can add any domain group that has been set up, not just individual users, or you can simply add all domain users, i.e. MYDOMAIN\Domain Users
to the Browser role, which seems to be what you're after.
However, I would recommend creating a generic user group like MYDOMAIN\SSRSReportUsers
or something like that and adding this group to the browser role instead of MYDOMAIN\Domain Users
, as adding all users to the Report Server seems like it doesn't give you many options to manage this in any sort of granular way.