I thought i would add this as i googled the crap out of it and couldent find nothing. This example uses Pidgin as the IM client.
1 - Firstly download the broadcast plugin
2 - create a new group and keep the name all lower case letters
3 - Add users to group, do not enable contact list group sharing
4 - In Pidgin go Buddies > Add Group and name it BROADCASTS or whatever you like
5 - Right click on the BROADCASTS group and choose add buddy
6 - Configure the new buddy with the full groupname.broadcast.openfireservername and give it an alias
Now you have a sub group in your contact list (in this case BROADCASTS) which contains distribution lists for mass messaging