Sometimes the color palette setting 'Success Button BG' is set to the same color as the page behind it. Making it unable to see the 'Add New User' button under the Profile > Users tab.
Check the color palette site setting 'Success Button BG' in the Pressero Admin under Appearance > Customize. You can try hovering your mouse on the storefront in the area where the button should display, this will trigger the 'Success Button Hover Bg' color. If this color is set to a different color than the background it should display. This most likely will confirm it is an issue with the color palette setting.