Unfortunately, there are no known physical group courses for learning Frisian in or around Rotterdam. Frisian is a regional language spoken primarily in the province of Fryslân, so most in-person classes are located there. Your options are online courses, self-study materials, or private tutors (in-person or via video chat).
To start, consider online group classes that meet live via Zoom or similar platforms. Many providers offer structured beginner courses with real-time interaction, which can give you the group dynamic you're looking for without requiring travel. Alternatively, look for a private tutor who specializes in Frisian; platforms like iTalki or local language exchange meetups (perhaps via Facebook groups or Tandem) could connect you with a speaker willing to do in-person sessions in Rotterdam if you find one. Be upfront about your location when searching.
A honest tradeoff: because Frisian has relatively few learners, dedicated beginner group courses are rare even online. You may need to combine multiple resources: a self-study book (like 'Frisian Grammar for Beginners') with audio materials from the Afûk (the official Frisian language organization) and occasional conversation practice with a native speaker. Some universities in the Netherlands offer Frisian modules, but those are typically in Leeuwarden or Groningen, not Rotterdam.
Concrete next steps: 1) Search for "Fries leren online cursus" and look for courses that offer live group sessions. 2) Check the website of the Fryske Akademy or Afûk for recommended online programs. 3) Post in r/Frysk or local Rotterdam Facebook groups asking if any Frisian speaker would be interested in a language exchange or small group. 4) If you specifically want physical meetings, you might need to travel to Friesland for a weekend intensive course, which some organizations offer.
Remember that learning Frisian via online courses is practical and often more flexible than in-person classes. The key is to prioritize regular speaking practice, even if it's virtual.