The songs (written by Keegan DeWitt) were actually performed by the actors with all the vocals being recorded live (i.e. on-set) with each take (similar to Les Miserables, etc.)
Dave the Bartender (Ted Danson) is said to have appeared on Broadway in the play Status Quo Vadis. Ted Danson actually appeared in that play on Broadway (which ran one performance in 1973), playing a bartender.
Nick Offerman and Ted Danson have previously worked together on the second season of FX's Fargo.
Jeff Tweedy was a guest star on Parks & Recreation, which Nick Offerman starred in.
Ted Danson played a bartender on the popular sitcom cheers where he also owned the bar much like in this film. At one point two characters raise a glass and say "cheers" while Danson is in the background. Later Ted Danson is referred to as "doctor" when he is pouring shots. He also played a doctor on "Becker"