Fusers always belong to one organization, we see it as the organization who owns the user
Fyou can then give that user permissions to other organizations, either with manager roles (ZITADEL internal permissions) or on projects (your app permissions)
Fits hard to recommend how you should structure it.
Fif a user registers with the private mail, i would generally go for a "general" organization, and than grant those users access to other organizations
Fbut if you have users from a specific company (org), also with their organization domain, i would recommend putting them into the organization directly
Fif an organization is deleted users from that organization are not able to login anymore