Final answer:
All males over 18, both citizens and immigrants, are required to register for the Selective Service System, which enables potential conscription into the armed forces.
Step-by-step explanation:
Based on current law, the group required to register for the Selective Service System includes all males over 18, both citizens and immigrants. This registration is necessary for potential conscription into the armed forces and all eligible males are legally mandated to register when they turn 18 years old. It's worth noting that there has been consideration to extend this requirement to females, although, as of the last update to this information, it is not yet in effect.