Who owns a project, who is invited to it, and what each side sees. This is the lesson that saves the most confusion later, because almost every question that begins "why can I not see" has its answer here.
What an organisation owns
Your organisation holds the subscription, the people, the tool catalogue and the templates. Set a standard once at organisation level, a workflow, a permission template, an inspection checklist, and every project you create afterwards starts with it. Some tools also exist at organisation level in their own right, including your own Directory, Documents and Equipment registers.
What a project holds
A project is the job. It carries its own directory, its own documents, its own contracts, and its own copies of the templates it inherited. Editing a project's copy never reaches back and changes your organisation's standard, which is what makes it safe to adjust a workflow for one job.
Inviting another company
You do not add an organisation to a project. You add a person, and their organisation follows them in. That keeps the list honest: a company with nobody on the project is not on the project. When you invite someone, you pick a permission template at the same time, so their access is settled at the door rather than discovered later.
What a guest sees
A guest sees the tools the host granted, and nothing else. Procurement, Reports and the project's Settings, Tools and Permissions pages stay host-side, so your commercial position stays yours. Guests still do real work: they answer mail, download the current drawings, run inspections assigned to them, and respond to compliance requests, all from the same screens the host uses.