Click on the Settings icon (the gear) located to the right of the search bar. This will take you here:
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In the horizontal navigation bar, select Events. A drop down will appear. Select Event Defaults. This will take you here:
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Within the setting box, use the On/Off toggle to the left of Assign Non Member Orders To Groups.
Once this is on, you will add Non Members/Guests to a Lieutenant/Group as normal.
Follow the directions here to do this.