If you decide that only admins can see other people's data, non-admins that log into NikaTime will only have access to their Overview page and won't be allowed to see anyone else's time. Otherwise, everyone in the team will have access to all the reports, with info about any projects and users.
If you decide only admins can create and edit team data, non-admins won't be able to create nor edit projects, roles, or groups. Otherwise, everyone will be able to create and edit that kind of data.
Admins can always access all of the team's reports without any restrictions.