I would argue that there are two reasons for the significance of the title The Joy Luck Club.
Firstly, it pays respect to the original Joy Luck Club, which was started by Suyuan Woo during the Second Sino-Japanese War. The first time around, it was a means for the main characters to keep their chins up amid unimaginable hardship. When Suyuan becomes part of another group with a similar spirit, this time in San Francisco with three other Chinese immigrants, it seems fitting to use the same name.
Secondly, the club brings joy and luck into the troubled lives of its four members, all of whom have endured much hardship. For starters, Suyuan lost her family and her twin daughters in a war, and settled in San Francisco without every knowing what happened to the twins. It is therefore evident that she is in great need of joy and luck, since her twins' fate only becomes known after her death.
The second member of the club, An-mei, has also gone through much, with her mother having committed suicide and her daughter having gotten divorced. The sense of community and friendship offered by the gatherings of these four women is one of the few sources of joy in her life.
Lindo has had ongoing conflicts with her daughter, Waverly, who is a chess prodigy, and is also in need of the joy offered by companionship with her friends.
Ying-ying's first husband abandoned her while she was pregnant, and her second marriage is loveless. Her second child was stillborn, which was naturally a source of great sadness. Her daughter, Lena, is in an abusive marriage, and since Ying-ying feels powerless to help, the moments she can share with her friends in the Joy Luck Club make all the difference.
In a nutshell, the significance of the title The Joy Luck Club is the historical significance that is has for Suyuan and the great need that all four main characters have for some joy and luck in their lives.