Only a member of a GitHub organization can claim its /dev/null page. The first member to claim it becomes its admin, and after that the page can't be claimed again.
Claiming asks for no extra GitHub permission: it just checks that GitHub lists you among the org's public members.…
Claiming only reveals what GitHub already shows publicly: that you are a member of that org. It doesn't grant /dev/null access to your GitHub account or change the org on GitHub. Worst case, if you leave, you stay listed as admin here and can't be replaced, so pick carefully who…