First run 'grep <username> /etc/group' to find out what groups your user already belongs to. Then run 'usermod -G <groups>,uucp <username>', as root. The group are separated by commas with no spaces. Omit the main group the user belongs to (ex. <username> or users). If the user only belongs to one group, <username> or users, you only need to run "usermod -G uucp <username>. For more information about usermod, 'man usermod'.
PS Make sure that group uucp owns your serial ports. I have no such group on my machine.