How do you add a site member to a group? - Click on "Members" tab in group.

How can I get access to node/nid/group?
It does not show up in the drop down.

Thanks for the pointer but it is not totally clear.

Let me re-phrase my question.

As an admin, how can I add people to either a public or private group?

There was an "add user" block for organic groups once.

Is "Find content" is located at admin/content?
Selecting the group does not give me the add user block.

How do I add users to a group?

For publicly available groups, members can access the group and “request group membership”

For private groups, following these steps:

Go to the “Drupal Admin” button in the top left hand corner

Select “Find Content”

Select your group that has been created by the site admin
Fill in the user you wish to add to the “Add Existing User” box

Ahhh, its under "Members" contextual_tab when you are logged in as an admin.
I would guess that the docs on the Openatrium page should be updated.