An elder wisdom of Marketing is: It's much easier and cheaper to hold an customer than to refer a new one.
So try to get a personal connection with them, thru weekly Teammail and especially three mails every two days to his signup-email.
Tell them that you want to give them succeed, you want to help them and at last give him a gift.
If there is no comment after those mails, forget them.
Take your power to other members.
Remember, you need only 5 who are working.
