As a flight attendant, I would have no problem with a group like this being on board.  The fact that it was a group of deaf  people wouldn't bother me.  If they were rowdy, unruly, obnoxious, aggressive.....then I may have an issue with them, but not  just for being deaf.  They may not sit in exit rows, there is a whole laundry list of restrictions on people who sit in exit rows, but that exit row restriction would be the  only distinguishing factor between them and any other group of people.
When FAs are boarded passengers with disabilities, we are required to give them an individual briefing specific to their 'type' of disability.  If a person is non-ambulatory (that is, they can not move about on their own) we tell them certain things, if they are ambulatory (they can move about on their own), we tell them other things.  When boarded a deaf passenger, we point out the safety briefing card, which clearly shows exit locations & operations, as well as other safety information.  When boarded a blind passenger, we offer them the braile briefing card, which they may choose to accept or not.
I do not know of any advance notification required to the airline.  I never worked in ticketing or reservations, so I'm pretty clueless as to that aspect of airline operations.
As a side note, I once had a deaf women on board, and I had no idea she was deaf until I looked closer at the passenger list halfway through the flight and saw the code next to her name & seat number.  This was  after I had done a complete beverage service, and had carried on a small conversation with her.  She could read lips & talk so well, I had no clue she couldn't hear me.  The agent didn't tell me about her, so I can only assume the agent didn't know either.