In the beginning of chapter 24, Scout mentions that Jem and Dill had gone to "Barker's Eddy" so that Jem could teach Dill how to swim. He had recently discovered Dill couldn't swim, which was "a skill Jem considered necessary as walking," so they had both left. Scout couldn't come along, because they were "going in naked", and so she had to stay put at home. So the first reason that she helps with the tea and the missionary meeting, is simply because she is there, and has nothing better to do.
One other possible reason is that Aunt Alexandra, since her arrival at the house, has been after Scout to behave more ladylike. Attending her missionary circles might be "part of her campaign to teach [Scout] to be a lady." Scout has to get all dressed up, help serve, and attend the "social" part of the meeting, in order to practice politeness and observe proper ladylike behavior.
So, a combination of unlucky loneliness and Alexandra's zeal to feminize Scout have led to her presence at the missionary meetings. I hope that helps! Good luck!
Aunt Alexandra made it her personal mission to make Scout more feminine as soon as she arrived in town. She sees Scout as a tomboy in need of a proper female role model, and to this end, she invites Scout to typically female social functions: missionary teas and the like.
It is within this realm that Scout is forced to wear a dress, much to her chagrin, and learns the societal expectations of a proper southern lady. She is also exposed to the gossip and business of other townspeople, and learns that missionary teas are nothing more than a chance for Maycomb County women to gather and dissect life in the small town under the guise of "Christian Fellowship."