How Do I Create a New Account/Identity In Photos App?

Hello there… I want to create an additional account in Photos. For example what I used it for before was I had 100 client photos and I did not want to mix them with mine. Some video I had seen showed me how to create another Photos account that I can use and switch back and forth from. It had something to do with the icon in the dock and a keystroke and click. Then it opened up a dialog box and allowed you to create a new account. Any ideas?
Thanks SO MUCH for all the time you put into helping people with their Macs. I’ve probably seen every single one of your videos!! Regards, Esther
I’m scanning over 300 family photos, want to add them to Photos, but don’t want to mix them with what I already have.
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Esther