If you're trying to reel in people and create a 'keep on playing' aspect, then you might have to sacrifice some of the "Castlevania-like" aspects.
It would most likely be a "I need help defeating the Melty Zombie. Send me a Subweapon Heart, please!", which would prompt people in your Vampire Hunting Guild to just click somewhere to help you.
It's taking a monster-slaying approach and applying the Facebook "Come help me you guys" thing that keeps people hooked.
Here are some ways it 'could' work, though most end up sacrificing the more core aspect of gaming. You need to find a happy medium where your game experience is still that of a monster slayer, and the social aspects are enhancements.
Something like this:
-your character is a hunter. He/she can hunt a number of things but is probably proficient at one, like...
--Vampire Hunter (great over all - some weaponry, some magic)
--Werewolf Hunter (specializes in weaponry)
--Ghost Hunter (basically a magic user, as weapons are useless against ghosts)
-your character earns levels as they progress through rooms of enemies.
--as your character gets stronger, he has access to some inventory of stuff to help him/her along.
-If your character enters a room with enemies it is not prepared to fight, he or she can request for help
--your guild can either have a friend join up (maybe someone competent) or have that friend send an item. If it were CV, you could send them a 'heart' or a magic subweapon
--subweapons are key here, as depending on what type of warrior you are, you may have access to only a certain set, sort of like how Trevor can use an Axe (physical) or a Cross Boomerang (A combination of physical and magical), while Sypha has an all-magical set (only her staff is half physical, half magical in this example). Magic items such as a screen-clearing rosary can be bought at a shop somewhere or can be sent to you for incurring massive damage. In this example, a Vampire Hunter is going to fight a ghost in the next room (you know which enemies will be attacking you in which rooms, so that the player can properly ask for help).
-You can try and fight the thing yourself, or 'leave' the game and wait for your powerup to arrive. If the powerup arrives, the hero will deal the appropriate damage while you're logged out. When you log into your game it'll check if you got the help you wanted and will perform the animation.
These are just some random thoughts about how it would work.
I'd love to get something like this going just using the Forum System in here.... but from what I've read, it's difficult to set up.