I am planning a big birthday dinner for around 20 people and was thinking about beef tenderloin. Any suggestions?

For a side dish for beef, I love puréed cauliflower. I braised a head, cut into large florets, in 1-1.5 cups of stock and milk. Bring it to a boil, lower the heat and simmer until tender, maybe 15 minutes. Purée in a blender or food processor. I like to add a 1-4 cup sour cream and 1-2 ounces of shredded sharp cheddar. Or you can so sour cream and horseradish with a touch of mustard powder. S

Alton brown makes an incredible beef tenderloin wrapped in a salt crust. It really does a great job of keeping the juices in and adding a great flavor to the meat.

I have two suggestions.
1. Hickory smoked tenderloin of beef.
Trim tenderloin from excess fat and grill on charcoal grill with indirect heat for about 35 to 40 minutes or till just under your desired internal temperature. Adding hickory chips atop the coals at the beginning of the cooking process. Remove the beef from the heat and let rest for up to 1 hour. When you are ready to eat, return beef to grill to reheat then slice and serve.
2. Beef ala fiscelle (poached beef tenderloin) This may sound unusual but is very good. Using a good beef stock flavored with red wine and herbs in a pot large enough to hold the tenderloin without over flowing. Poach to your desired tempature and remove. Rest beef at least 20 minutes. Keeping stock hot until you are ready to serve. Return meat to pot, heat, remove and carve. Serve with horseradish mouse. (sour cream, whipping cream, horse radish, L&P, tabasco, S&P) Whip the cream and fold in the other ingreidients and you have a light horseradish flavored sauce. Good Luck

If I were you, I'd do a main beef dish that can be prepared a day or two in advance and even tastes better that way. 20 people is a lot and you will be stressed enough getting the drinks and appetizers out and actually managing to take a shower and look presentable. I'm thinking Boeuf Bourguignon http://food52.com/recipes/2969-beef-bourguignon
Or short ribs
http://food52.com/recipes/8578-short-rib-ragu
Those both are delicious and look like you went to a lot of trouble, but they are actually quite easy. You can cook them the day before and then reheat in a crockpot, on the stovetop, or in the oven while the party is going on.
However if your heart is set on tenderloin, here is a good recipe
http://food52.com/recipes/14725-porcini-and-rosemary-crusted-beef-tenderloin-with-port-wine-sauce

Once you decide on the protein, the rest will be easier to plan. You don't want to have too many things in the oven at once.

You could go for the classic... beef Wellington! It's a great dish for a special occasion--it's got that 'oooo-aaahhh' factor. Plus mushrooms are looking food right now and will shortly be at peak production time.