This plugin can be enhanced significantly to further reduce time required for adding existing user to blog
1. by giving additional setting to allow admin to set default role, say subscriber so that every time it is set on page load. It is torture to set user role from administrator to other one manually, most commonly being used are subscriber and author. No one is going to bulk add administrators!
2. Please provide drop down list for email, so that they can be added quite easily. OR ajaxify so that they show up on typing. this would reduce copy paste from other sources.
3. Still better would be to use username, rather than email, as username is never changed after signup but email can be. another reason is that username is publicly available but not email ID