Can I join the dark brotherhood as a werewolf?

I'm a werewolf, does this prevent me from being abducted and joining them?

No, you can join the Dark Brotherhood, even if you're a werewolf.

Since you have completed Innocence Lost, you need to wait for a few days and enter a city (Example: Whiterun). A courier will give you a note that says "We Know". And then sleep in a bed and you'll be kidnapped.

if you speak to Arnbjorn, he reveals that he is a werewolf, meaning The Player is not the only werewolf in the DB.

No it shoudldn't I've done almost the same same, getting in the companions Circle and becoming a werewolf after joining the dark brotherhood.

In fact being a werewolf gives you an aditional speech whith one of the brotherhood which is indeed a werewolf (Astrid's Husband).

Quoting the wikia

He is an ex-member of the Companions. He was asked to leave because his "unorthodox" methods were offensive to other members. It is unclear what these methods were or how they were offensive, however, given his current status, it seems possible he used underhanded, duplicitous techniques that would have offended the Companions' sense of honor. He joined The Circle and became a werewolf prior to his expulsion.

He initially disapproves of the Dragonborn, showing a rude attitude, but after several successful assassinations, he begins to respect and even envy the Dragonborn.

If the Dragonborn is also a werewolf, however, he will immediately know and react a bit more amiably. Regardless of whether they are a werewolf or not, Arnbjorn will use little nicknames such as "lambshank," "beef roast," "tidbit," or "hamhock." He says not to take offense from the nicknames.

So, being a werewolf does not prevent you from joining the dark brotherhood.

Finally, coming to your issue, have you tried going to sleep in your house ?

Quoting Wikia again:

24 hours after Innocence Lost has been completed, a courier will appear to the Dragonborn in any city, with a note saying, "We know." A black hand-print is above the words. After receiving the note, sleeping in any bed for any amount of time will begin the next phase of the quest (the acquisition of this note is optional).