Short answer: In the Netherlands, BIG-registered therapists may not offer psilocybin, LSD, or MDMA as regular mental healthcare outside approved research or another specific legal basis. Working with truffles is not a regular GGZ treatment.
A BIG professional can take part in non-medical psychedelic activities only outside their BIG role and with a clear, provable separation. Do not claim therapy or treatment, do not use the BIG title for promotion, do not include current or former clients, and keep organization, communication, and records fully separate from the regular practice.
Disciplinary risk rises when roles mix. Authorities judge each case on its facts, so caution and clarity are essential. When in doubt, get independent legal advice.
If you are exploring a non-medical group setting, you can read about a four-day psilocybin retreat here: four-day psilocybin retreat.
I hope this clarifies it.