It is clear that 10 people shud be called at a time for max. no of parties.Now,let us consider a group of ten friends which consista of a friend A.Now,keep A aside and select other friends (9+10remaining) to make diff. groups.This can be done in 19c₉ ways.In all these groups,a particular friend A will be there.So,the ans is 19c₉ ways.