[1] He studied law at University of Groningen (the Netherlands) and at the Westfälische Wilhelmsuniversität Münster (Germany).
In 1982 de Groot started to teach at Maastricht University and was subsequently appointed as a professor (retired in 2016).
[8] In 2016, de Groot with Olivier Vonk published the book entitled International Standards on Nationality Law: Texts, cases and materials,[3] Archived 2021-07-09 at the Wayback Machine with the assertion that while grant and withdrawal of nationality - done based on domestic law - has been traditionally considered a matter of State sovereignty and discretion, the developments in international law over recent decades have produced a growing body of “international standards” setting limits to such State discretion.
The book offers the first comprehensive collection of international treaties and instruments, caselaw and other materials.
[9] Since December 2016, de Groot is a member of the Committee of Experts of the Council of Europe, monitoring the implementation of the European Charter for Regional or Minority Languages.