Give feedback when invited-by-email user is already a site member

[boonebgorges]

From user report:

Also, if when they use email, and the user was already a member of the site, rather than just removing them, it would be more seamless if you prompted the user to confirm in-site invitations right then and there (or cancel) for such invitees, rather than just tossing out the names,requiring that the user go back to the previous step and invite them as existing members. This saves steps and confusion. If it's supposed to work that way, it didn't in our tests. But again, this can be pretty confusing.