The best way to keep fajitas warm for a party is to wrap the fajitas in tin foil and then place the fajitas in a warm oven.
The oven will keep the fajitas warm but turn the oven off after about 30 minutes and keep the oven closed and then the foil should keep the fajitas warm enough until you're ready to serve them.
If you're making the fajitas at home and taking the fajitas to a party then you can keep the fajitas in the warm oven until you're ready to leave.
Then place the tin foil or aluminum foil wrapped fajitas into an insulated lunch bag which will help hold in the heat.
You can also buy a 12 volt lunch box that keeps the food warm until you're ready to serve it.
You can place the fajitas into the 12 volt lunch box and plug it into the 12 volt outlet in your vehicle and keep the fajitas warm until you're arrive at the party.