There are a few different ways you can add users to workspaces.

Solution 1

The easiest way is through the client itself, but before are able to you must have satisfy one of three conditions 

1. you have ownership of the workspace
2. you are a delegate of a workspace 
3. you are given membership permission rights to a workspace

Once you satisfy one of the above conditions you can simply right click on the member pane of a workspace and select "Add New Member"

Solution 2

You can also add users to workspaces through the admin tool by following the below steps

1. Login to the Admin Tool
2. Expand the Organizations Node in the left hand Navigation Tree
3. Right Click on the Organization the specific workspace you wish to add a user to is located under
4. In the context menu click the menu item "Show Content"
5. On the dialog that displays in the left hand content area, Click on the specific workspace you are working with
6. Click the membership button near the bottom of the dialog
7. From this screen you can simply add or delete members using the right and left arrow keys