Two people can only be on the same PNR is they have the EXACT same itinerary.

If you upgrade and the other person does not at the same time, the PNR will be split.

You will hear about "linking" PNRs. It is next to meaningless and amounts to nothing more than adding a comment to the PNR referring to the other PNR.

You can sponsor one other person into E+ with you. Not more than one, even if on the same PNR.

If the other person is not on the same PNR as you, but you are both on the same flight segment, I think you can still sponsor them up to E+, but I am not 100% certain.