I am looking for a word or expression to describe those ladies, usually high society ladies, whose main aim in life is to find a good match for marriage and whose main activity is organising lunches or dinners for relatives and friends often to raise money for some non-profit organisation?
I've always referred to them as "ladies who lunch." According to Wikipedia the phrase means:
"Ladies who lunch is a phrase often used to describe well-off, well-dressed women who meet for social luncheons, usually during the working week. Typically, the women involved are married and non-working. Normally the lunch is in a high-class restaurant, but could also take place in a department store during a shopping trip. Sometimes the lunch takes place under the pretext of raising money for charity."
"The phrase "ladies who lunch" was introduced in the January 19, 1970 issue of New York magazine by the writer Merle Rubine,1 "Anyone with a fair figure, ready cash, fashion savvy and a safecracker's nerve can buy the best that Fifth Avenue has to offer on Seventh Avenue at half the price. The girls at Condé Nast and Harper's Bazaar have known this for years. Likewise the ladies who lunch at Restaurant X, although they'd rather be banished from the banquette than admit they got their Beenes and Blasses on a bargain basis."
For the first part of your question, consider debutantes.
debutante, (informal) deb: a young upperclass woman who has begun going to social parties where she will meet and be seen by other people from the upper class.
clubwoman: a woman belonging to a club or active in a club or other social, community affairs.
mondaine: a woman who is belonging and well known in fashionable society.