Are my friends immune to the harmful effects of radiation?

It doesn't actually hurt the companions they just mention it a lot so you don't have to worry about it killing them or something. For role-playing purposes you could leave the human companions & dog at home for areas like the glowing sea or put the human ones in power armor if you are one of those that likes role-playing like that which is what I do. Here is a link where they discussed it not many comments though. https://steamcommunity.com/app/377160/discussions/0/492378265877233598/

No NPCs have radiation damage applied to them from the environment (gamma gun and etc is exempt), with the exception of feral ghoul enemies. Environmental radiation heals them. Your companions do not "knock out" due to environmental radiation damage or receive healing effects.