When you drag a Person/Group Picker onto your form, the underlying structure is a repeating group as shown here:
You may allow the picker control to contain multiple selections by editing the control's properties:
When you allow multiple selections and you want to create rules based on someone in the list, you must perform the "contains" logic on the group folder. If you just look at the AccountId or the pc:Person folder, the logic will only be "true" if the account you are looking for is the first entry. Performing the logic on the folder that contains the pc:Person repeating group, allows you to include all person/group entries in the checking of a specific account.
This may be used in rule conditions:
Or it may be used in formulas using the contains() function:
You may use this logic to hide/show sections, controls, etc. based on the current user being selected in the Person/Group picker and/or allow additional functionality.