After encountering Solaire of Astora in the Undead Burg (after the Taurus Demon fight but before the Red Drake encounter), initiate a conversation with him and answer "yes" to both of his questions. Killing Solaire where he is first encountered will also yield this as a drop should the player not acquire it prior to killing him.
Using the white sign soapstone causes the player to place a friendly summon sign on the ground, enabling other players to summon them for co-operative play. In order to use the soapstone, players must be online. White Sign Soapstones may be used regardless of whether the user is hollowed or in human form, or whether they had personally defeated bosses themselves.
Players can only see white summon signs if their owners are within 10 levels (+/- an additional 10% of their current character level). For example, a level 100 player can choose to summon anyone between level 80 and level 120, and a level 30 player can choose to summon anyone between levels 17-43. Being in NG or NG+ has no influence on this (players in NG can summon others in NG+ and vice-versa).
It is impossible to use summon signs (or any other online item) when another player is in the process of invading the user's world. This can be used to an advantage as it gives hosts an early warning and time to prepare.