Arrange several tiki torches in the area where your guests are congregating. Fill the torches with citronella oil and keep the wicks lighted during the entire time your event is happening. Make note of any changes in wind patterns and move the torches accordingly.
Encourage your guests to spray a DEET-based repellent on their clothes, as opposed to putting it directly on the skin.
Change all of the light fixtures outside from regular white light bulbs to yellow light bulbs, which have a repellent effect on mosquitoes.
Grow some catnip in your garden or in small pots and place the plants strategically around your yard. Catnip and catmint have natural repellent properties.
Use commercial devices that emit carbon dioxide to attract mosquitoes away from your gathering spots. Female mosquitoes bite humans and animals because they are attracted to the carbon dioxide and cholesterol found on the surface of the skin.