Go to the Blackbaud NetCommunity plug-in in The Raiser's Edge, click on the "Options" link.
Once the pop-up window appears, select "Linked User Email" select an email template and save.
The "Send Linked User Email" button should re-appear

In the event, no email template is available for selection,
Log into the Blackbaud NetCommunity website and go to email templates to create a template that is Data Source:  NetCommunity Users

Only Email Templates with Data Source: NetCommunity Users is accessible for use within the Blackbaud NetCommunity plug-in in The Raiser's Edge.