Once you've rung your Beckoning Bell, you can pull up to two other players into your game to help out. However, once you've connected with another player, a Sinister Bell-Ringing Woman will spawn in your world who will attempt to summon a player to invade you. If you can track down and kill her, you'll eliminate the possibility of being invaded until you initiate another co-op session. Later areas in the game will spawn the Bell Women regardless, making the most difficult areas even more stressful.
Summoning specific friends in the Souls games has been… difficult, to say the least. After dealing with the witchcraft of timing logins and specific locations of the original Dark Souls, Dark Souls 2 introduced the Name-Engraved Ring, which let you filter potential partners down to people who chose a specific deity to worship.
In Bloodborne, you can password-protect your session, which will only allow people using the same password to connect to you for co-op. It's not without its issues, as connections can take several minutes to complete (and a connection error can cause you to disconnect entirely), but you'll be able to roll with a steady crew if you want to. And in case you're wondering - no, this doesn't prevent you from being invaded.
Progress is only saved for the host player, but if you've tried co-oping one of these games with friends before, you'll be used to leap-frogging progress for everyone.
Though I consistently played with Andrew, Jason, and Locke, there were times I tried seeking out random players to fill out my group, with mixed results. While I was summoned into other players' games relatively quickly, if I wanted to get someone into mine, it was often an exercise in patience, taking five to ten minutes to match up. This tells me that - at least at the current point in time - the player base is less focused on jolly co-operation and more concerned with making progress in their game.
The Oaths also leave a lot to be desired, unlike the multiplayer covenants in Dark Souls. First, there are only three, and two of them are opposed to one another, so initiating a co-op session and pulling a player of the opposite faction makes you hostile to one another, rendering the whole point moot. Secondly, there doesn't appear to be any kind of reward for participating in covenant activities other than the Oath Memory you acquire and the odd character emote, which is quite unlike the old Covenant system. Third, there's no equivalent of the popular "Sun Bros" to encourage co-op play. This is a pretty glaring issue that I feel will detract from the longevity of Bloodborne multiplayer.
During the course of play, you'll find Chalices that allow you to unlock Chalice dungeons - these are randomly generated and filled with loot and bosses not found within the core game. Completing Chalice Dungeons earn you materials to create more complex and difficult dungeons to tackle. These can also be played with friends, and particularly memorable/loot-filled ones can be shared to the wider playerbase. They make for a good diversion if you're having trouble making progress in the storyline, or if you're looking for something else to do after completing it.
Bloodborne is a fantastic game, and if you're looking to play with friends, it's certainly the most easily recommendable game in the Souls series. Yharnam is a joy to explore, the combat is thrilling, and the rush you get from defeating bosses is as great as it ever was. The Chalice Dungeons and New Game modes should keep you busy for quite some time.