When the Styleguide plugin is enabled, it is expected that only the groups that are set in the styleguide_allowed_groups will be able to access it; the admins are enabled by default.
When I try this on 7d5cb8ed, I can access the /styleguide page anonymously, and with any logged-in user.
Tested locally using:
Discourse 3.6.0.beta3-latest - https://github.com/discourse/discourse version 7d5cb8edc0b2589e3a05f0935ef8365e9bea5397