You cannot select users from multiple pages of a Multi User Picker field.

1. Click the people icon to pop up the name picker.
2. Put a checkmark next to any name.
3. Goto page 2.
4. Put a checkmark next to another name.
5. Click "Select"

Expected result, each user that I have placed a checkmark next to is added to the notify box.
Actual result, only the user(s) on the last page I viewed before clicking "Select" are added.

This means to add multiple people, I would need to open the picker box several times in order to repeat for every page-worth of people. This should be changed so that you can select multiple users from multiple pages.