For complimentary premier upgrades, United explains how they are handled for itineraries with more than one passenger:

https://www.united.com/ual/en/us/fly.../upgrades.html

For elites, an itinerary for multiple passengers will remain together until ~T-24. If the upgrades clear before then, the PNR remains together. If they have not cleared by then, the PNR will be "split" when you check in for purposes of upgrades, with waitlist priority in accordance with above. Note: If there are only two passengers on the PNR, then United no longer splits the reservation automatically, and instead only if one (or both) of the passengers are upgraded. https://www.flyertalk.com/forum/unit...-upgraded.html

The splitting has consequences regarding eligibility for "inherited status" - i.e., baggage allowances, upgrade priority, boarding priority for passengers with lower status than the highest status passenger. I.e., as a formal matter the benefits of inherited status are lost (although E+ seats are usually retained or can be gotten back, baggage is usually granted, one can board with highest priority passenger). This also matters for the return.

This thread explains various scenarios and considerations as to whether it is desirable to split the PNR before T-24 and the consequences of having it happen.

Questions to consider:
1) Do you mind if not all passengers are upgraded? If you are willing to have some people remain in coach, then splitting increases your chances of an upgrade before T-24 because sometimes only one upgrade seat becomes available at a time. In such scenarios a group of 2+ will be skipped. If you can't be separated, don't split until forced to do so. If you really can't be separated, consider a call to UA to ask to be marked not for upgrades.
2) Once you get inside T-24 your PNR will be split, unless you make a special request that it not be. Each passenger will be placed on the upgrade list in accordance with the above. This doesn't matter much on the outbound flight unless there are IRROPS, in which case it may be tougher to get accommodation for all passengers.
3) On the return flight(s) the companions may no longer have the highest level of status of all passengers, including baggage allowances (E+ seats are usually retained). However, some recent reports are that companions retain the status of the highest passenger on the original PNR, even after it is split.

related threads
The stupid PNR auto-splitting at T-24 needs to stop

https://www.flyertalk.com/forum/united-airlines-mileageplus/2039553-no-more-splitting-pnrs-get-upgrade-list-but-still-split-if-upgraded.html

Splitting up partner airline award reservation

Archive thread : To split or not to split PNR {Archive}