GEMA fees are only due for the public performance of music. An event is considered public whenever admission is charged, and also when guests are admitted who do not have a personal connection to the hosts.
For example, your garden party with close family is not public, even if the whole neighbourhood can hear it. Turkish weddings, Polter weddings or Polterabend celebrations, on the other hand, are public even when they take place in a hall, because by tradition anyone may join in. Funeral receptions are public unless they take place only within the closest family circle. School graduation parties for pupils, both parents and the closest teachers are not public. But if tickets are sold in advance, as is often the case for a prom, the event becomes public, because it is not the organising committee but the ticket purchaser who determines who attends.
The organiser of a public event must register it with the responsible GEMA district office in good time and pay the fees.
In addition, the DJ must also obtain GEMA licensing for copies of the music used at public events. I have already taken care of that.
If you’re still unsure, just ask me.