Tell-a-Friend forms are not permitted for use with AWeber.
What's A Tell-a-Friend Form And Why Can't I Use Them?
The way that they work is that someone enters the email address of one or more other people that they believe would be interested in the information being offered. Those people (the "friends") are then sent one or more messages from the publisher.
The problem with such Tell-a-Friend forms is that the person whose email address is entered never requested that information, and as a result, any email sent to them would be spam.
A Better Way
What you can do instead is to put a section in your message with instructions on how to sign up for the list.
That way, your subscribers can forward the message themselves to their friends, rather than generating a message sent by you through some sort of form. If the friends want to sign up for the list, they can do so by following the instructions in your message.
What's the Difference?
When your subscribers forward a message from their email account, it is very clear to the recipient that this message is coming to them from the subscriber - someone that they know.
When someone adds another person's email address through a Tell-a-Friend form, the recipient may have no idea why they are being contacted, and the message they receive comes from you - someone who they don't know.
It's also important to keep in mind that when people get email they believe is spam (whether or not spamming was the sender's intent), they are going to hold the sender of the message - if you use a Tell-a-Friend form, you are responsible if someone signs up a friend and the friend reports the message as spam.