Most parts of the Members Area, along with the forum, are restricted to those who are paying a membership-fee (or who received an exemption). Areas related to internal democracy are additionally restricted to those who have verified their identity.
There are also areas that are restricted to people with particular admin privileges, for example only DSC coordinators can access the "Review Members" page to see who joined or might like to join their group, only NC members can upload crowdfunding pages or petitions, only a small number of admins can access the member administration, and so on. If you need to do something and you're not sure who has the access to do it, write to firstname.lastname@example.org.
Finally, there is restriction conditional on people's profile data. For example, some votes are limited to MeRA25 members (if they are deciding about a party matter or electing party representatives). Also crowdediting and questionnaires can be restricted in this way, in order to prevent noise. If you have a livestream or some secret information that should only be visible to DiEM25 members or a subset of DiEM25 members, an admin can put it on a Restricted Page for you. This feature is quite new, so contact email@example.com for help in using it correctly.