There are a number of good sites that talk about the skills and communication of group riding. Try the www.bikeforce.com.au
I use the expresson 'Bike' when passing, and indicate which side I am passing on. This is an accepted practice.
To point out an obstacle, it is as simple as pointing at it, and saying what it is. A car behind is simply that! Say it loud and be heard, respect is earned in a group by good timely communication. If your slowing, braking (minimise where possible, sit up if you can and let the wind slow you) shout it out.
When overtaking someone, using the hand for the side of the road there on, bring it around your back as if your trying to touch the middle of your back with the back of your hand, all done in a nice sweeping action, it is trying to indicate to tuck in behind, as there is something to avoid on the side the arm came from.
I'll try and get out to one of these rides with you guys and show you the techniques, they really are commonsense.
Oh and when your going up hills, when you stand up to make it easier, be aware, that the action causes the bike to lurch back a little and someone behind might tap your wheel, and generally they fall.
Watch the road ahead of you, don't stare at the behind of the rider in front, stay near the front of any group, otherwise you get the yo yo effect. Don't be afraid to ride at the front and set the pace, even if it is only for 50 metres, you'll be greeted with appreciaton for it, and no half wheeling, what's this, the bug bear that causes the most probs in group riding, when the rider besides you in two's, seems to want to ride half a wheel ahead, it means you try to get level, then they go back to the half a wheel ahead and so on, and this causes the group to split up, which is not the purpose of group riding, team work is the goal.