Change handling of "already member" addresses

Currently, if you try to invite someone to the site that is already a registered member, you get an error message. I'd like to propose that if the user already exists, rather than returning an error message, we go ahead and create the group invitations and give a success message: "User already belongs to the site. Group invitations have been sent" or similar. In the case of no group invitations, I'd like to return the message as a success banner rather than an error banner, since it's not really a failure.

Thanks!