Programming :: Text On A Specific Line At The End Of A Line?

I'm trying to add text to a file for a specific group of users, I'll need to do examples as I can't think of an easy way of explaining, my file is like this:

Code:

users{
user1
user2

[code]....

At the present my code lists all the available groups, how would I add a user to a specified group? (e.g add "members user3") to the end of group 1 for example. So the code ends up like this

Code:

members user2,user3