There are a few other tags that can be used in your FTAF form and envelope to help you personalize the message:
|%%$first%%||First name of the friend|
|%%$sender_first%%||First name of the sender or subscriber|
|%%$sender_last%%||Last name of the sender or subscriber|
|%%$sender_email%%||Email address of the sender or subscriber|
The last three items are all taken from the subscriber record. If you decide to use something like %%$sender_first%% in your envelope, you will want to make sure that all of your subscriber records include a first name. If they don't, you can include some logic that will display alternate text in case the first name in a subscriber record is blank. For instance, instead of:
"Your friend, %%$sender_first%%, thought you'd be interested in this."
You may want to use:
"Your friend, %%$sender_first%%%%if sender_first = "" then printdata sender_email%%, thought you'd be interested in this."
The second example will use the email address of the original subscriber if his or her first name is not included in the subscriber record.