As soon as people start to put more time in the water, they invariably end up swimming in more crowded conditions, whether that be in your group practice, or in the lap swim hours at your local pool. When swimming in a group, it is great if everyone follows the same set of "rules of the road", or rather "rules of the lane". It is very, very normal to have people swimming at different speeds, even when we have our workout group sorted by speeds. Whenever the sets involve more than 25 yard repeats, people start to get lapped, or people tire and the people behind them begin to catch up, need to take a little extra rest and miss a 50 or so, or someone gets confused and finds themselves off synch with the set that everyone else is doing. The longer the distance, or the least amount of "regroupings" that occur make this more likely: if you are doing a long continuous swim, or a "set rest" schedule, by the end of the set, people will be scattered about the lane.
In all of these cases, you will invariably find the need to either 1) pass someone, or 2) be passed by someone. When doing this, it is great to have a set of pre-agreed upon procedures for passing or being passed, so that everyone knows what to expect. This will make your workout safer for sure, but it will also make it so much less frustrating. When a group of individuals adhere to the same best practices they can work like a well-oiled machine in the pool, allowing the competitive spirit and comradery of a group workout to truly take hold. Be aware, be consistent, and have fun.
Rules of the Lane
When being passed:
Never, ever stop in the middle of the pool -- wait till the end of the length, then swim directly to the right-hand corner and allow the person to pass you.
Swim closer to the lane rope so that when you get to the end of the length you will be out of the way already.
Touch the feet once to let them know you're there, then try to chill out until the end of the length, when they stop to let you pass
Turn in the center of the lane, or towards the left-hand corner, leaving the right corner for the person to stand while waiting for you to pass.
Passing on the fly (not recommended):
If someone is swimming past you in the lane, swim as close to the lane rope as possible
If you are passing someone, swim right down the middle, do not cross all the way over to the other side of the lane to pass them.
Communicate with your lane mates at the beginning of every set -- order yourselves according to speed, fastest first.
Always move to the right-hand corner of the lane when stopping (assuming that you're swimming counter-clockwise)
Always try to flip in the middle of the lane or towards the left-hand corner (assuming that you're swimming counter-clockwise)
When starting a length after a rest, check for people who are swimming in to turn, if they are faster than you and close to the turn, wait a few extra seconds to let them go by before leaving. It is better to have to add 5 seconds on to your interval at the start than to take off without looking, only to have to stop 25 yards later to let someone pass -- it will interrupt your rhythm as well as theirs.
Always circle swim: If you are in a workout group that normally has 3 or more swimmers in a lane, even if it is a sparsely crowded day, circle swim anyhow. This way, if anyone shows up late, or if someone from an adjacent lane moves over into your lane, it will avoid confusion and possible collision, since everyone will know what is happening in the lane.