This relationship must be initiated and accepted by an admin on each side. Assets can be shared/un-shared at any point, however, this does not remove the association with the recipient business. Once the relationship between the two businesses is established, it cannot be undone.
The partner who is being granted access will receive standard Business Notifications and as well as an email notification. They can go into Business Manager to accept the share, and there is no penalty for not completing the flow, other than an inability to share a business tool.