You can invite anyone you like. If these people do not yet exist as persons in innius, they will receive an invitation to create a company account within innius.
The action for inviting others is itself subject to authorization, which means you can determine exactly who may invite others.
Next, the other company must first be flagged as a trusted company before you can share anything with them. They will then have a very limited default authorization policy assigned. You are in control of what that other company may do.