Awaiting Decision: Contributor-Based Access (User vs Company Access) to the External Dashboard - Part-2

**Ticket created to complete the remaining work carried over from #1356 whenever a decision is made**

As an external user who has a BCAP login/account,
I want the dashboard to show files in the system that were created by me and also by my company,
So that I see only authorized information, and also able to take one a file of someone else as long as they are in the same company as me.

**Acceptance Criteria**
- [ ] User can be linked to one or more companies from list of companies

Start by reading the remaining-work context from issue #1356 and the unresolved decision about user versus company access. Clarify the authorization rules before locating the dashboard and account-linking entry points; done means the agreed behavior is implemented and the acceptance criterion for linking a user to one or more companies is met.

Written by the indexing model from the issue text.