After addind _many_ users to a single group adduser will stop appending the
new user to the end of the line in /etc/group and will start putting a new
line in after each user.

This script will show you the problem:

for i in `cat /usr/dict/words` ; do adduser -G users $i ; done

the "users" line in /etc/group will look like this:

users:x:100:user1, user2, user3, user4, user5, etc, etc, wholalottusers
,nextuser
,nextuser
,nextuser
,nextuser

  Adding each word in the dictionary will get to the C's before it starts
this behavior, so it takes a few hours. Adding a user manually through
Linuxconf while your adduser script is running will hurry the process
though.