One of the core features of Intervals is time tracking project management. When users track their time in Intervals, the time entries are aggregated into a weekly timesheet which can be submitted for approval. All Administrator level users are notified and can approve submitted timesheets.
To see users that have not submitted their timesheet(s), as an Administrator level user, navigate to Time > Missing Timesheets.
From the Missing Timesheets page, pictured below, you can send email reminders to your team asking them to log in and fill out their timesheet. Check the boxes next to the people to notify, or notify one person at a time using the Notify button.
Filtering options are also available, if you need to narrow down your search.