You can use AIM codes in hyperlinks to interact with the AIM client.  Here is the list of AIM codes and how to use them:

To use a space
If you want to use a space be sure to place the + symbol where there would normally be a space.
Example: message=Hello,+this+is+a+test! Click Here

Opens a new IM window
aim:goim
Example: Click Here

Opens a new IM window with a screen name
aim:goim?screenname=SCREENNAME
Example: Click Here

Opens a new IM window with a screen name and a message
aim:goim?screenname=SCREENNAME&message=MESSAGE
Example: Click Here

Opens a chat room
aim:gochat?roomname=ROOMNAME
Example: Click Here

Opens the "Add Buddy" window with a screen name to add
aim:addbuddy?screenname=SCREENNAME
Example: Click Here

Opens the "Add Buddy" window with a screen and a group name to add
aim:addbuddy?screenname=SCREENNAME&groupname=GROUPNAME
Example: Click Here

To change someone's buddy icon when they click on the link
aim:buddyicon?src=BUDDYICONURL
Example: Click Here

To open or receive someone's shared files
aim:getfile?screenname=SCREENNAME
Example: Click Here