Getting Mass offered - some questions

I would like to get Mass offered for each member of my immediate family… including my grandfather who passed away several years ago. I was wondering, what effects does offering a Mass have on someone who is living? Is there anything I should do as well, like say some special prayers, or does the priest just offer the Mass in his intentions? Also, if my grandfather is in Heaven, would the graces obtained through this go to other souls in Purgatory?

Thank you :slight_smile:

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