Eminem was at one of his usual watering holes, Hot Rocks with his wife, Kim Scott. It was after closing time outside the club when a dozen people who recognized Eminem approached his party, and then all hell broke loose. 

The details are shaky at this time, but a fight occurred after Eminem was approached.  The bouncers from the night club jumped in to break things up. When one of them saw a gun had fallen to the ground, the police were called out to the scene. Which resulted in the arrest of four people including Eminem. 

Charges were filed against two of the people for suspicion of carrying a conceled weapon and the other two were charged with disorderly conduct. The Police did not make public which charge Eminem faces. 

Update:
As of June 6th, Two of the four arrested have been formally charged. Eminem's wife, Kim Scott was arraigned on a charge of disorderly conduct. Also arraigned was Robert Brown, who pled not guilty. Eminem has not been formally charged, but it is believed he was one of the two individuals arrested and then later released for suspicion of carrying a concealed weapon. If convicted Eminem faces up to five years in prison.