The most common reasons you can't find a user in the Add Users dialog box are:
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The person is already assigned to a role in the project - Each user can be assigned to only one role within a project. To change the role that a user is assigned to, see How do I give a user a different role in a project?
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The person is not a Smart Build Insight user - If the person is not listed, it means they are not a Smart Build Insight user. To fix that, see Give a person access to Smart Build Insight.