As a Silver you get Business Class lounge access for yourself only.
As a Gold you can bring one guest into the Business Class lounge as well.
As a Diamond you can bring two guests into whatever lounge you can use that day (First if flying OW, Business if not).

If you got guests into the First class lounge not as a Diamond then that must have been as a courtesy. I have no idea how often that happens since, as a Diamond, I don't need it.

Note that if you are a OneWorld Emerald then you can bring one guest with you into the First lounge when flying OneWorld.