Anyone got some information on bees...?

I was sitting outside and suddenly a huge swarm of bees flew over my head and drifted around awhile. After around two hours, they settled in an African Sumac (not sure on spelling) and made a big clump on the trunk. The next day, it almost looked like they had added parts to the tree, as if they were making a honey comb for a hive, but I cannot see under them to what they are doing. I'm not sure what exactly I should ask, are there any bad effects the bees might cause to the tree? We have a large vegetable/herb garden nearby, and the pollination might be useful, but then again we live in the middle of several almond orchards (lots of bee boxes in the spring). So should we just bring someone in to remove the bees? Also, we are about to do a lot of landscaping around the tree and they might get in the way, but so far they have not bothered us. I know what people always say, don't bother them and they won't bother you, so any information would be helpful, thank you :)