Know your honey
First thing to do is please DON’T PANIC. Swarming bees are the most docile bees you’ll ever get to meet and they are not interested in stinging you. Please don’t let anyone kill them with insecticides. Just sit back and watch them be the amazing creatures they are.
The second thing to do is find out if the insects you are looking at are actually bees. There is no point in calling a beekeeper if you’ve found wasps, carpenter bees or bumblebees.
If you’d like to know more about swarms and what bees are doing when they swarm I recommend you read the book Honeybee Democracy by Thomas D. Seeley.