How do I add Android user (linux level) to a group? groupadd and useradd aren't available, so how I can add a user (app's assigned user) to a group?

Also, the file /etc/group does not exist and I couldn't find where Android saves this information. (googling/greping).
I don't have an android phone in front of me, but I imagine you could just edit /etc/group and add the user to a group. Find the group you want to add them to and insert a comma, then the user name. 

Example : 

sambashare:x:115:sam,sally,yourNewUser


Where the first column is the group name, then the second column is group passwd (leave it an x), groupID and then the users.