Can anyone offer me some advice please?

I've been asked to take photographs of guests at my sister in law's wedding. I'll set up my D5100 indoors on a tripod near a window to give natural light. However I would like advice on which lens to use. At the moment I only have the 18-55 kit lens and don't have the option to spend loads on a new portrait lens. The photos I'll be taking will be mainly posed couples.

Thanks in advance for your advice.