How can I generate a random alpha-numeric string and have it plugged into a login/signup box for a group that is on the same (activity) page

I want to have an activity that has a login button to make a random alpha-numeric string and when pressed populates a single login box on the same activity (page). This will be used for their user name/password. Then they just press the login button and it will create/signin a group named that string in Parse. Once the last person leaves the group I would like it torn down (deleted from parse.) That's is another problem though... I can get the random number generated and the login box created but connecting it all is my issue. (I am new to programming, like green as can bee maby a few weeks old.) I