Q: Can I specify additional groups etc when creating a user account in CentOS?

 

A: Yes.

User will use group number 20 as initial login group:

# adduser user -g 20

 

User will have home directory set to "user-home":

# adduser user -d /home/user-home

 

For more info see: 

http://linux.die.net/man/8/adduser

http://www.unix.com/man-page/Linux/8/adduser/

http://www.tecmint.com/add-users-in-linux/