In order to manage users you need to have the Manage Users permission. Check out the Reviewer roles / permissions article to see what roles have that permission. To get started first make sure you are inside of the correct project and click on settings icon.
This will bring you to the project setting section. Click into the Team Members tab. This will list all the active/pending team members that have access to the project. Click on the user who needs to get the invitation email again.
Click the Resend Invitation link and that will bring up a dialog box to confirm. Click yes and the user will get the invitation email.