At the top of any item or discussion, you can see all those who have access to it. Their names are shown to the right of your profile picture. All items within a single discussion - Tasks, Documents, and comments - will be visible to everyone who has access to the discussion.
This article shows how you can edit this access.
To edit access, click the Share button immediately below the start of the discussion or item. If you are in the Feed view of the 4th Office, this will be immediately visible to you. If you are in the Activities or Document view, follow the instructions in this article to find the Share button.
Click the Share button and a dialogue box will open. This will allow you to add or remove other Users, Groups, Projects or Customers.
If you wish to remove access, simply click the small grey X next to the User or context you wish to remove, then click Share.