People form different companies can collaborate on a project as long as their access level doesn't use the restriction they can only see their own company, group, or team. (And if you have people from different companies, you can even have "private" conversations on the project, tasks, issues)
Yep! You just might have to use the "Run Report As" feature with an admin so everyone can see everything on the report.
Depends on how you share things. The only real restriction I know with Company is that you cannot have a manager from another company. (Which is why we have FCB Global Network for the company and utilize Groups to do separate agencies/business units)
For us, companies are "the client" so our custom form has information from our Financial System that we use integrations. We have what is the Client Number in SAP and then we have which "Company Codes Have Access" which is which Agencies bill to this client.
As I mentioned, we use group for the agencies and company for the "client". This is helpful in Healthcare especially since the portfolio is at the Brand level and a single client might have its brands across multiple agencies. We haven't gotten to the point where clients can come into our Workfront instance, but we do it this way as well, so if they do, they can be restricted to just their company's projects and (as mentioned above) there is functionality for our teams to communicate in the tool without involving the client.
Seconding everything Anthony said, and a few other thoughts:
Custom forms by company can be useful if you're pulling data into consolidated reporting systems - ex we have our accounting system's unique ID, plus a few high level groupings that let us mirror reporting out of WF the same way it would be in the financial system etc. We also provide a URL link to the company in our Salesforce system in case company names change, easier to track back a discrepancy.
We do have customers logging into our Workfront instance, and the company field is key to properly ensuring that they have access to their information/forms and nobody else's.
