select dropdown for belongs_to field

Hi,    I have a 'person' model that belongs_to a 'group' model. I used scaffolding to generate the form code for adding a new person, but I want a select drop-down for selecting which group to assign a person to. I looked at the rails form helpers guide, and tried a few different things in the template, like:

<%= :vendor, @vendor %> but i'm getting errors...

what's the proper way to do this? is there any controller code necessary?