How to create groups and assign users to them using Organic Groups? I have downloaded the Organic Groups module for my Drupal project and am currently trying to create groups and assign users to them.

I don't want anything fancy. I just want to be able to create groups, have the user assigned to them by the main administrator (not allow the user to register themselves) and have the assigned 'leader' of the group be the only one within the group with edit/etc permission.

I am unsure how to start. I found the resource Organic Groups: Enable users to create collaborative groups that looks really good, but I think it is for an older version of Drupal because the OG sections I have found are in a different location than the site says. 

How do I do this?
Have you read the readme file in og module folder, it shows basic instruction to create a group, assign content to the group. 

Basically, after enabling OG module, you need to create a content type, if you want it to be a 'group', select the "Group Type" option, then each instance of this content type will represents a group.  

Below are some links will help you start:

Basic OG Documentation: http://drupal.org/node/483234

Amitai's presentation of OG7: http://www.archive.org/details/GrouptheNewOrganicGroups-BuildingSocialNetworksInDrupal7    (I recommend you watch this video, very informative. I am sure you will sort it out)