[Knurps]

function foo:DebugGroupJoinedEvent(event, name, displayName, isPlayerLocal)

is short for

function foo.DebugGroupJoinedEvent(self, event, name, displayName, isPlayerLocal)

so now the event code will be stored in the variable self,
the name will be stored in the variable event,
the display name will be stored in the variable name etc.
That's also why your debug output prints "event: Exoy^Mx".

you want to use

function foo.DebugGroupJoinedEvent(event, name, displayName, isPlayerLocal)