Regardless of how long your friends have played or not played, the ONLY way, on vanilla game, to have bosses spawn at your base is to use console commands... something that can be "learned" in a matter of minutes, from watching videos or reading a guide.

Bosses, do not actually show up for raids, they send their minions to raid...

I have played through all the current content, numerous times, ALL vanilla and all solo, and I know for sure that bosses do not spawn at players base, ever, for any reason.

One of your friends DID do this and now it's your choice to be in denial or not, but one of them definitely did this.