Welcome to the FusionAuth community! I am glad you like the software.

You can't create group fields. What you can do is assign roles to a group. If a user is in that group and registered for an application with those roles, they will assume the roles. For example:

User A is in Group B. Group B has roles 1, 2, and 3. Role 1 is in Application Z. Role 2 and 3 are in Application X.

If user A is registered for application X and is in Group B, they will have roles 2 and 3. They won't have role 1 because that is associated with application Z, which they are not registered for.

You can't build your own entities, though that is on our roadmap. Follow this issue to be updated on when that feature is implemented. Full transparency, it will likely be a feature limited to our paid editions; learn more about our paid editions here.

Whether groups make sense depends on a lot of things.

Some questions to consider:

Are you limiting application access by group? Are you going to be using the API to do so, or do you want to have the information in the JWTs and available after authentication? Does each company have an application? Are you going to use the hosted login pages or build your own?

I can't give extensive architectural advice, but maybe if you talk a bit more about your use case the choice will be clearer.