You need to be subscribed to these users' presences. If so, you will receive Presence updates from them.
However, this is more a server issue. E.g. I think Openfire does not allow to be subscribed to a user without adding this user as a contact to its own roster. And if you send a subscription request to a user, Openfire will automatically add this user to your roster, even though you did not add it to your roster.
