I originally wrote AIM Broadcaster as part of JAMS back in 2005. When I broke out JAMS into separate plugins Broadcaster was left out, but now it is back as a separate plugin. Download the executable, exit AIM and install the plugin. Restart AIM and to send out a broadcast message to an entire group, just right-click on the group and select ‘Broadcast IM to this Group.’ NOTE: In AIM 6.5 there is a bug where the group you select from the right-click menu will not be the selected item in the dropdown menu on the broadcast form. You need to select the group in the dropdown. This bug will be fixed by the client in a subsequent release. You can download AIM Broadcaster here.
Here is a screenshot of the broadcast form: