Ubuntu 11.04, adduser myuser: admin group is missing

use groupadd admin to add the group then you could run adduser myuser admin. However you may want to instead use usermod -G myuser,admin myuser to remove yourself from the root group. (instead of myuser,admin, list the exact list of groups your user should be a member of.

If you ever get tempted to edit /etc/group directly, please use the vigr command, which will make sure the syntax of the file is correct before committing changes.