Having shared a record, you can add or remove 'share-ees' and/or you can add to or amend the comment you entered when you shared the record by editing the shared record listing in your Shared by me screen.
Records shared BY me
Go to your 'Records shared by me' screen by clicking on the 'Shared by me' link in the Collaboration section of the left side navigation bar to view all records you are currently sharing.
Click on the edit icon on the right side of the shared record listing you want to amend.
The edit pane displayed is similar to that which is displayed when you share a record.
If you want to remove a 'share-ee', simply click the 'X' on the unwanted user's name in this field.
If you want to share the record with additional users or groups, click in the 'Shared with' field to display a list of available users, then after entering the first characters of a new name, select the additionally required users and/or groups from the displayed list of matching names.
The select box remains open enabling you to select multiple names. Click outside the select box to close it or click again in the Shared with field and enter a new value to display new matching names.
When you have finished, click on the tick icon to save your changes and to return to the 'Records shared by me' screen - or click on the cancel icon to discard your changes and to exit from edit mode.
on the right side of the shared record listing you want to amend. 
