Good grief. So apparently, the right to petition the government is only "sort of" an individual right.
In the First Amendment, no words define the class of individuals entitled to speak, to publish, or to worship; in that Amendment it is only the right peaceably to assemble, and to petition the Government for a redress of grievances, that is described as a right of “the people.” These rights contemplate collective action. While the right peaceably to assemble protects the individual rights of those persons participating in the assembly, its concern is with action engaged in by members of a group, rather than any single individual. Likewise, although the act of petitioning the Government is a right that can be exercised by individuals, it is primarily collective in nature. For if they are to be effective, petitions must involve groups of individuals acting in concert.(Emphasis added)
So, because you're unlikely to be all that effective petitioning the government on your own, that's not "really" what the right was meant to address. This is important because Stevens' analogizes this "sort of" thinking to the 2nd.
Similarly, the words “the people” in the Second Amendment refer back to the object announced in the Amendment’s preamble. They remind us that it is the collective action of individuals having a duty to serve in the militia that the text directly protects and, perhaps more importantly, that the ultimate purpose of the Amendment was to protect the States’ share of the divided sovereignty created by the Constitution.
Stevens, then, appears to take the position that you have neither the right to petition the government or to fight to insure "the security of a free State" unless you have a bunch of people helping you. In essence, he's arguing that because you're unlikely to succeed without help, that this is the only legitimate way to try at all.