L.D.
I dont think you have to pay. Send his coworkers and email saying that you are taking your husband out for his birthday and ask if they would like to come too. That is different than saying you want to take them all out for your husbands birthday.
I work in an office where this happens every once in a while and I would definitely not expect for the spouse to pay for everyone.
Bring a cake or cupcakes for everyone to enjoy.