Adda is a Hindi word meaning... well, it's tough to say what Adda means because we don't really have a better Hindi or English word for it, so here is the definition I picked up from Wikipedia, and I guess it is quite to the point.
Adda is a noun with the nominal form of the word meaning the location or nest of a group or community, usually shady people. The etymology can be traced to the original meaning of the word, which means the "perching spot or perch for birds".
Sadda Adda here would be 'our adda' or you can say, our spot, as Sadda is Punjabi for our(s).