Hi @luiss.dimatteo,
Unfortunately, the only way to get a list of users in an Organization is by using the Management API to get an access token to make a request to the Get members endpoint.
The best approach is to use the Management API for this scenario. While you might be able to leverage ID tokens, user metadata or custom claims to achieve this, it might reach its size limit if you have too much data stored.
Thanks,
Rueben