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You can add a person from a different company/group to a project by simply checking the check box next to the name of the person in the list to the right of the pop up window, which appears while adding or manging a project.
P.S- The company of the person you wish to add should already be a part of that project.

Can I keep certain team members/clients from seeing specific projects? How do we set permissions?
You can give anyone permission to access any project, or keep anyone from seeing any project.This can be done by checking the checkbox next to the name of the person which appears under the list to "Add contacts to this project". This list is seen in the popup window which appears when we add or manage a project. The person you wish to include in a project should have his checkbox checked and vice versa. Thus we can set permissions.
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