Okay I know this may sound dumb but bare with me, I can't get it to work or find an example anywhere.

I can get the UserId of someone, but I want to get the username from that user ID.

any idea how, and please post examples i learn better that way.

Do you mean in the templates?

if you mean on template you can use $bbuserinfo[username] on template for 3.0.x, for 3.5.x i think it's $userinfo[username] .
if you want on query, via code, just run query like:

For 3.5.x it's still $bbuserinfo[username]

For php code I use $vbulletin->userinfo['username']

Yea I am doing this in php

I think i can do it how mad dog said.
basicly run
that would work, would it not?

EDIT:
Of course I have to add the mysql access code

You not even have do that query. What do you want to get from the users table?

He wants a User's username when he has the user's userid ...the simple query above would work but it might be better to incorporate it into an existing query to reduce the mySQL calls ;)

If you want the username of the person browsing the site use $vbulletin->userinfo['username']

I don't want the person browsing the site, I have a user id, drawn through the $_GET array, and i want to find the id's user name
i am gonna try the mysql query theory and see wht happens

agreed - point well taken.
it's always better to add extra field or locate some query that already has it for better optimization. on the other hand, 1 small query that have only 1 result filed in it & query that won't return any big array should not take much resources & would be very un-noticable in most cases.