Below are the basic conditions required to summon other players within Dark Souls 

Conditions for summoning (White Sign Soapstone):

1. The host (person who's game world will be used) must be human. The client (person joining the game) can either be human or undead.


2. The client’s level must fall within 10% (above or below) of the host’s level (this is more strict in the earlier soul levels). 


3. The area boss (Taurus Demon, Bell Gargoyles, Moonlight Butterfly, etc…) in the host’s world must still be alive. A client can still be summoned even if the client has already defeated their own area boss.


4. The host must not have placed down a summoning sign. Summoning signs from other players will not appear in a host’s world if the host also has placed a summoning sign down. This is to prevent a network conflict of the host joining someone else's game world while the clients are trying to join the host's game world.


5. After successfully summoning, the client will return to their world if the area boss is defeated, the client is killed or the host is killed.