Nikon Lens Suggestions?

I'm taking pictures for a wedding fairly soon. I would like to be better equipped in terms of camera equipment so I can deliver the best pictures possible for the happy couple. I'm having some trouble deciding which lens I should get. I have a Nikon d3000 camera body. I have the 18-55mm kit lens. And I have the 50mm f/1.8 lens. Which is what I mainly shoot with. But this lens does not have a focus motor with my camera body, therefore I must struggle through the manual focus. Which is fine and I can do it, but it just takes a bit and how I would hate for the whole wedding party to lose patience with me. So I'm wondering if anyone has any suggestions on lenses. Possibly a good lens for portraits, group shots, or candids. I don't really want to spend more than $500 so this might be a bit of a challenge. But any help is appreciated!
I have looked at an 85mm lens and also the 18-135mm lens. Let me know what you think of those. Thanks!

Added (1). Please NOTE: I'm tagging along with a profesional on this one, so I would just like to get a general idea of what lens would be good. No need to tell me I should not be taking wedding photos if I don't even know what lens to use. I just really need someone's help. Thanks

Added (2). Also please note, the professional has given his two pennies worth but I would like more opinions. I do not mean to bother anyone because it is simply of the individuals own accord that they chose to open this post. Sorry if you feel bothered…

If you have to ask what lens you should get to shoot a wedding then the truth is you shouldn't be shooting a wedding. A wedding day can't be reshot and if you do not have the skills or knowledge to do it you should leave it to the professionals.

Ask the pro then, why bother us?

You do not have enough gear to shoot a wedding - my basic gear is;

- two DSLR bodies (Nikon D100 & D300)
- 50mm f1.8
- 85mm f1.8
- 24-70mm f2.8
- minimum two speedlights, plus modifiers & triggers
- plenty of spare memory cards & batteries

although DSLR bodies are pretty reliable, you need a backup in case anything goes wrong (it can happen, & 'sorry' doesn't cut it with the bride on the most important day of her life).

You don't sound anywhere near experienced either - you want to work as a second shooter for at least a couple of weddings before going it alone.

Edit: sorry I was typing as you added your edit. You didn't mention it in your question, so people are bound to assume you are the principle shooter. Fair enough then! I would choose either the 24-70mm f2.8 (Sigma do as reasonable version) or the 85mm f1.8 - however, both will be manual focus on your body.

Edit 2: The reason we 'bother' on here is to try to prevent newbies who are clueless shooting a wedding & making idiots of themselves and ruining someone's day. You clearly do not fall into that category!

"Also please note, the professional has given his two pennies worth but I would like more opinions"

WHY? Our opinions are worthless for this.
The pro who you are assisting (tagging along with) is the ONLY one who can tell what you should be bringing.

It may be that they want you to do specific shots which require just a single prime lens.

First: Considering RENTING a lens if you are not sure you want to spend the money for it. There are lots of online lens rental stores.

Here's what I'd look for in a lens for weddings, portraits, and available light:

A FAST 85, 100, or 115mm length lens. (Maybe a zoom in this range--they can be handy if you don't want to bring an arsenal.)

Fast refers to the aperture number (lenses like f/2, f/1.8 would be fast, f/4 or greater would be slower).

A faster lens is vital if you plan on doing any available light and/or indoor work. Your images will turn out dark, blurry, or noisy (too much noise from high ISO) if you are shooting in light that is too low for your lens; and a standard built-in flash isn't very flattering--nix the on-camera flash as much as possible.

85mm and longer lenses are also much more flattering (for human subjects) than shorter lenses and you may have better depth of field options (more of the pretty background blurriness while keeping your subjects in focus) with a longer, and faster lens. This can help make beginner style photography look more professional--kind of an easy "cheat" to get good photos.

I took the photographs at a friend's wedding yesterday afternoon and found the 24-120 f3.5 / 5.6 G lens covered literally everything from close-ups though portriats and interior church shots including at the altar and in the vestry. I abhor flash and used a high ISO of 4000 ASA for my interior shots which, on my Nikon D700, gave perfectly good results. As I write I'm currently taking a break from converting the images from RAW to JPEG and expect to have about 70 acceptable images for the bride / groom to look at.