If a lane is empty, you may enter that lane and swim.
If all lanes have swimmers in them, ask someone if you may join them. Don't enter the lane until they know you will be joining them.
Simply talking to your lane mates about who should go first or what type of stroke each of you are going to swim will help keep peace and harmony in your lane.
Oh, and NEVER dive into a lane with swimmers in it. Instead, sit on the side and gently lower yourself into the water.