Raid events have a chance to trigger for each player if said player is near at least 3 base structures at the time (not counting the "you are being hunted" raid, that can happen anywhere). Unattended bases do not meet those requirements and will usually be fine (unless another player who IS online happens to live close enough to keep the area loaded and possibly in the event radius). The downside with raids in multiplayer is that the raid progression is based on the current highest player progression (as in, if your friend is already up in the mountains when you are just starting, you can get raids well beyond your current progression because SOMEONE on the server is that far along, even if it isnt you specifically).