Yes, multiple people can "E-muster", also known as self-muster, using a single phone. A phone is technically not needed to self muster.
The new E-muster process is still a work in progress and the procedure may change, but at this time your personal phone (Carnival Hub App) is only used to give you the location of your muster station (which can also be found on your Sail & Sign card) and does not play any additional part in the self-muster process. Once at your muster station each of you will have your Sail & Sign cards scanned and be given a quick safety briefing on life jacket use.
There will also be a short safety briefing viewable from your cabin TV. A mandatory safety briefing will also be broadcast over the ships pa system (at which time all onboard services will be suspended until the briefing is completed), you do not need to be in your cabin or at your muster station for the mandatory safety briefing. The mandatory safety briefing broadcast lasts about 6 minutes.