A militia in 1789 did not require membership. A militia was simply all the able-bodied men, all of whom were expected to show up with a personal weapon to defend their localities in the event of invasion. Every man was expected to keep a gun in his home for this purpose. That means that this right to bear arms applies to everyone.

In fact, this is similar to a jury and being called for jury duty. You have no special training in legal affairs, yet you are expected to show up in business attire and decide the fate of someone accused of breaking the law for very little compensation.