Advice for riding in tight groups

1. Hold your line. learn to pre-empt obstacles and hop small holes rather than swerving at the last second.
   
2. Agree who's leading the group and follow them, both direction / pace and advice. This may be one person or several experienced riders
   
3. if your following someone's wheel, don't just assume they will look out for obstacles. use your own eyes and ears too
   
4. Relax
   
5. (if you have gears) copy the cadence of the other riders, it'll feel much more "right"
   
6. Let others know if you're changing pace or overtaking. A clear "on your left / right" if you're coming through, for example.
   
7. Respect the experience of better riders. You may be fast but they will have more miles under their belt
   
8. Warn others of hazzards. do it clearly and quickly, without wobbling and causing another hazzard :-)
   
9. if the group splits, it's up to the group leader to get the group back together. Usually this happens at the top of the hills
   
10. Let the experienced riders manage traffic and act as a backstop. don't try and play traffic cop without the group leader's ok.
    
11. take your turn at the front, but don't be a hero
    
12. make sure your bike and you are prepared for a long ride. you won't be popular if your bike fails due to a simple maintenance problem
    
    
    

none of this mentions the basics though, about being able to ride on a wheel in the first place.

Look ahead, not down at the wheel in front, look several places up the line, or over the shoulder of the rider in front.
Do not overlap wheels, if the rider in front has to swerve he will take your front wheel with him.
Always look over your shoulder before making any moves.

Re; fixed and gears.
a mixed group will always be difficult to keep together, as the freewheelers will tend to run away on every descent, this is less of a problem on the flat and the responsibility falls with the geared riders to adjust to what the fixed guys are doing. I've ridden in groups of very experienced riders with a mix of gears and fixed with no problems before.