Regarding indulgences (which are always a good consideration) you can receive all the Rosary indulgences by praying only the Sorrowful Mysteries as often as you like.
The rule has long been that all 5 decades of any THIRD of the 15 Mysteries are required for one Rosary indulgence. So you could pray only the Joyful Mysteries or only the Glorious Mysteries as well as only the Sorrowful Mysteries. There are no Rosary indulgences for praying less than a group of 5 Mysteries in one day. All you would receive is the indulgence (if any) for each of the prayers you say.
All 5 mysteries must be completed in one session, in one day, with reasonable interruptions if necessary. If you stop in the middle of a particular decade you should resume later in the same day by starting that decade over with the Pater Noster. When praying alone you do not need to literally announce the mystery, so long as you can do the meditation for that mystery -- but for most of us it helps to speak the name of the mystery. When praying in a group of two or more, the name of the mystery must be pronounced, generally by the person leading the prayers. Reading or spontaneously speaking a brief meditation of that particular mystery is often very helpful for the group, but sometimes it can be a distraction or a cause for disagreement (the devil is always looking for ways of interfering with our prayers).
I was privileged to attend a daily rosary group that met in Beverly Hills at the parish Church there many years ago and there was a very gracious older lady who led the prayers. She had a seemingly endless variety of brief meditations of about 20 seconds that she would say before each mystery. I asked her one time after the prayers where she gets these descriptions of the 15 decades and she told me that she had long been practicing a spontaneous meditation for each decade, and it comes naturally to her even without having to try hard. She thought that she was following the lead of the Holy Ghost, Who gave her the words to use. I never heard her say anything contrary to Church doctrine in those prayers. I told her I was very impressed by her contribution and it gave me something to think about for each decade, more than I was accustomed to having. She replied that anyone can do this, as it is a product of asking the Holy Ghost for the grace of contemplation on the Rosary Mysteries. I would expect she is no longer with us, as she was likely in her 70's at the time and that was about 1984. So now she would be over 100.
Curiously, only the original 15 decades (by name, officially) have been given indulgences, and the 2002 addition of the "Luminous" mysteries never was given any indulgence.
When you are in a group and everyone is in the state of grace, each person praying the Rosary receives the indulgence of all the people present. Of course, you can only receive one plenary indulgence in a given day. But if there are say 4 people, each person gets 4 partial indulgences, and perhaps one plenary. If the group is 50 persons, each person gets 50 partial indulgences and perhaps one plenary.