Bottom line
You can belong to both your State's Naval Militia and the Marine Corps Reserve at the same time. Congress allows and encourages it, but it is not required. While you are on active duty, you are excused from all Naval Militia service and duty. This section also includes Figure 12-1, which lists the main benefits and privileges for the Retired Reserve, split between those awaiting pay at age 60 and those already drawing retired pay.
What it means for you
If your State has a Naval Militia, you are free to join it on top of your Marine Corps Reserve membership. The two do not conflict, and your active duty obligations come first. If a State ever mobilizes you, you can wear the Marine Corps uniform, but you must wear Naval Militia distinguishing devices so nobody mistakes your status. The Figure 12-1 list at the bottom is the quick reference for what you keep and what you gain when you reach the Retired Reserve.