For this one I managed to complete it on the first day with the first 8 survivors. I was going for the Alpha and Omega achievement as well when I did this.
First get Denyce
Then go kill Ted and either save or kill Snowflake. If you save Snowflake then you will need to take him/her back to the safe house, make sure Denyce is far enough away before you go in. Snowflake will be placed there no matter how far away s/he is.
On your way out of the safe house pick up LaShawndra in the Dark Bean and George in Casual Gals.
After that go back to the Yucatan to get Lenny. He is by the service desk in the employees only room.(this also nets you the Powered Up achievement)
***During this time if you are going to be doing the cases to keep the survivors away from the safe room. Having them stay in the maintenance room is a good place to keep them. Press LT and Y to tell the survivors where to go, they will go that location and stay until you call them again.***
Next follow the "Know when to Fold'em" mission to get Bill
Next mission should be "Welcome to the Family" with Jack and Kenneth
And then "Lush-ous Lady" with Kristin.
The location of the clown car for me was outside the Wedding Chapel in the Silver Strip. Get them all in the car and the achievement is yours. I hope this works out for others, as I said it is how I got this achievement.