Hi Jacqui,
Unfortunately it's not possible to filter by groups in the participation report. The problem of the course participation report not respecting separate groups is reported in the tracker as MDL-21415 and you'll notice I've added a comment mentioning this discussion. Please vote for the issue if you'd like to see it fixed.
In the meantime, perhaps you could use the send message feature on the participants page to bulk message all users in a particular group.