These are all informal words not defined anywhere. But we call all companies are uder a group, associates, affiliates etc when its under the control of same promoters may be by virtue of shareholding or directorship
Group companies are companies promoted by same promoters and/or any promoter group i.e relatives as defined under co.'s act. It is not defined anywhere but it can be recognised by directors/shareholders pattern or even the common name of group used, if any.
Associate company is also same but it is called associate when there are any transactions between such group companies which are known as related party transactions under the act.
The term group company is not defined. It simply means a set of companies controlled by a common apex company. To call a company an associate of another, the another company must have shares in the first mentioned company atleast 26% but not exceeding 50% (AS 18).