Anything's possible with the Questing botbase. You just have to know how to do it.
You can go the traditional way, and pass a "RunMacro" to directly invite the player.
eg: <CustomBehavior File="RunMacro" Macro="/invite Playername" />
Or, you can call a Lua function
eg: <CustomBehavior File="Misc\RunLua" Lua="InviteUnit('PlayerName'); " />