In order to collaborate with specific, internal users of SpreadsheetLIVE, you must know the specific user's SpreadsheetLIVE log on name and the user must have updated their SpreadsheetLIVE status to "publicly accessible".
In order to grant users collaboration permissions to your document you need to select the spreadsheet document from your workspace and then use the right-mouse click button to access the context menu and select Manage Sharing > Share (as shown in the image below).
In the Manage Sharing dialog box you will need to enter the users SpreadsheetLIVE log on name (or at least part of it) and then click the Search Users button. A list of users matching the search criteria will appear, select the user or users that you wish to collaborate with and use the arrow (> or >>) buttons to grant the users access to the workbook. Once you have granted users access to the workbook you can choose whether you would like the collaborators to have Read or Write access to the workbook. Collaborators will be able to recieve real-time updates regardless of their access level but they will require Write permissions if you would like them to be able to add to or modify the document. Once you have made the appropriate selections remember to click Save Sharing to apply the changes!
At this point your document is shared (you will notice that the icon beside the document has changed to indicate that the document is being shared). The collaborators can now find your shared document through the Workbook Menu > Shared link on their workspace page.