from the man pages
If called with one non-option argument and the –system option, adduser will add a system user.
By default, system users are placed in the nogroup group. To place the new system user in an already existing group, use the –gid or –ingroup options. To place the new system user in a new group with the same ID, use the –group option.
A home directory is created by the same rules as for normal users. The new system user will have the shell /bin/false (unless overridden with the –shell option), and have logins disabled. Skeletal configuration files are not copied.