The standard method of doing this would be to use the 'Find and Edit' section of the 'Students' tab within the LabTutor Server Admin. This method can be quite laborious for a large list of students.
A more effective method would be to simply re-import the list via 'Students' --> 'Import'. The 3rd Step of the process will indicate which Students do not exist on the database (they will appear as 'Additions' if they do not exist on the database, and 'Updates' if they are already present on the database):
You then then just click on any other tab in the left-hand menu to cancel the process. In the above example, the Students already exist as they are flagged as 'Updates' rather than 'Additions'.