What about someone who just joined recently ? How can their average be calculated ? If the avgnew of the last meeting is taken as the avgold of the next meeting would it cause imbalance because the first "avgold" has an influence in how the averages go from there on ?

I think taking avgold from a set of last meetings ( say 6 ) before a meeting and then taking 0 for absence to get avgnew. This way the average level would be reflected more accurately.