I have not edited the group/raid buttons, but I think it is as easy as making a new line and typing the macro there
So I added a tell to the Gravitas button that tells the target of the spell that he/she has it and also a tell to the raid to say who it is on.
Code:
<Data Name="Button4" Macro="SpellForMyLevel='Gravitas'
useabilityonplayer Parent.Target SpellForMyLevel
tell Parent.Target Healing stuffs for you
raid Gravitas on Parent.Target" Tooltip="(80) Gravitas"/>