By default users are added directly to a project team and notified that they've been added. However, if you run a project where you are looking for a higher level of commitment from your project team it is a good idea to turn on the Invites feature in your project template.
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This can be enabled in any existing project template or set up for any new templates. Writing an onboarding message is required so new team members know what to expect when joining projects using this template:

Once enabled any new users invited to a project team will see an invitation banner at the top of the page and will be met with the onboarding message if they accept:

If a new team member instead declines the invitation they have the option of leaving a message making it clear why they had to decline: