When many people first join a club they find riding in a group somewhat daunting.
I'm talking here about simple group riding, not "through and off", "pace-line", "bit and bit" or whatever you want to call it - the stuff that you'd do under race or team time trial conditions. This is just the bread and butter long training ride or clubrun stuff.
Well, it's common sense really.
The routes of our group rides are all chosen so that it's reasonable to ride in twos for 90+% of the ride. Riding in twos is perfectly legal in most situations, but often it will be courteous to single out to allow following traffic to pass. Those at the front may not know if there's a car behind so those at the back need to shout.
While riding in twos is usually reasonable other road users will find the sight of riders in threes (or more) highly irritating. Indeed if the group is not in military style formation it will look from behind that the "group was all over the bloody road" to coin the pub conversation. So, to prevent confrontation, stick to twos and keep all the gaps even.
Typically, we all should be within 1m of the guy in front. Riding a few cm outside or inside the line of the rider in front will give you more of an escape route if they slow down suddenly. If you ride >1m behind then you'll not be getting much shelter and the guy next to you will have a dilemma. If he rides in position, then the formation is shot, if he rides with you he's working harder than necessary.
Riding 20m off the back all day is asking for trouble as it is unlikely that everybody else will realise if you're held up for any reason (traffic lights, puncture etc).
Call out hazards such as pot-holes, parked cars, horses etc to those behind.
Avoid doing anything suddenly, especially braking.
Try not to jump out of the saddle onto the pedals on climbs. This causes the riders centre of gravity to move forward and the bike moves back to compensate. The rider behind then has to take evasive action, followed by the rider behind them etc. With practice you can actually take about 2 pedal revs to get yourself up on the pedals and minimise the backwards lunge of your rear wheel.
When you've done your turn at the front it's time to go to the back. If the road is very quiet then check behind first, then the rider on the left moves in, the rider on the right moves out and the group passes through the middle. If it's a bit busier then the rider on the right moves in front of the rider on the left, they slow and the bunch moves past. It's often tempting to not slow down very much which means the bunch takes a long time to pass - not good as you'll be > 2 abreast. This temptation is usually to avoid the little sprint to rejoin the back of the bunch, so if you are at the back you can help your mates by shouting "last man" as the returning riders slide back past you.
Half-wheeling is the phrase given to riding half a wheel in front of your mate on the front in an attempt to encourage them to ride quicker and quicker. This is bad form, but usually a sub-conscious act. So if your mate is doing it to you then tell them (politely the first time). If you see it happening to somebody else then point it out. And at the end of the day if you mate keeps doing it to you then just let them go - he'll look daft 5m in front of you and soon get the message.
It's no good riding faster than the whole group can manage and its counter productive to have the weakest rider hanging off the back and therefore having to work harder. If you see someone struggling then tell the front riders to "knock a mile off" (slow down a bit).
All the above is really applicable on flat to rolling terrain. On the bigger climbs the group will fragment and you should regroup at the top. When restarting make sure the weakest rider doesn't push off last as then he's likely to have another little sprint to perform to regain the shelter.
Given that the group will break up on the big climbs its fair game to race your mates, but if you do this on every little pimple between the Square and the Cafe you'll not be popular. It can also be fun to sprint for town signs and the like, but again maybe not every single one unless this has been agreed up front....
In general, a rider that is finding it hard is the least able to influence how the group is riding; breathlessness or pride tend to inhibit a shout and when it does come it will be given with some emotion! Therefore it's really everyone's job to watch out for the signs that someone is under too much pressure and then to act.
Don't get dragged into slanging matches with motorists as you'll drag all your mates into it as well and one day someone will have a gun! Just let it go - you can never think of the best response until the adrenalin has subsided anyway!
When a car approaches from behind don't wave them through even if you are sure it's clear. Make them make their own decision. What can often happen is that they'll hesitate and dilly-dally then move out and by this time it's no longer safe. If they then get into difficulty with an oncoming vehicle; 1) we'll all be involved in a potential accident situation and 2) they'll point their finger at us for waving them through.
Get some third party cover by joining either the CTC or BC - similar cover, similar cost. But you'll need to join BC if you want to road race anyway.