To illustrate how to allow a user to access projects in multiple organizations, follow this example. Let's say that you want to give Joe Bob access to 2 projects - each under a different organization.
1. Check which organization Joe is in by going to "People & Organizations" in the main navigation -> selecting the "People" tab -> finding the user -> and going to "User Organization". Here we find them in "Organization A".
2. Ensure "Organization A" has access to projects from organization A & B by going to the "Organizations" tab and selecting "Project Access".
3. Give Joe access to the correct project by going back to Joe's page found in the "People" tab and selecting "Project Access".