I've been playing with the APRS functionality on my Kenwood TM-D710GA, one of the settings is for message groups. If I understand it right, this means I can send an APRS message to the group and all group members receive it.
Note that this is different from bulletin groups.
Is anyone else interested in using NWOL as our APRS group message call?
I believe that the only to send an APRS group message is through APRS-IS. So, you'd need to be able to connect through a server and have it relay the messages through it.