There are major characters (those who keep appearing in our lives - for better or worse) and minor characters (those who pop in and out of our lives at significant times).
One way to get a grip on the enormous number of people who we have come in contact with during our lifetimes is to categorize them as major and minor characters. Write down a list of major characters - for now, just write down 4-6 of them. Then write a list of minor characters - these people who are important, but pop in and out of our lives during times of happiness, conflict or change.
Then rate them - put a plus or minus sign in front of each person - some will have both a plus and a minus and often these people are super significant and need to be included in your memoir. Then write for five minutes introducing your major characters to each other at a party.
Do the same thing for your minor characters and see where it goes...there...you have begun your memoir writing. Give yourself a pat on the back.