Members of the team are assigned to issues based on their Project Issue Availability settings, which can be found under “My account” on http://security.drupal.org/user
After you enter your availability, the project_issue dropdown displays only those issues that are appropriate for your availability. The actual assignment is currently done by the team coordinators, although if there is an issue that interests you, feel free to assign it to yourself.
If an issue is assigned to you and you don’t plan to work on it, please unassign yourself. If you can find someone else to take it over, that’s even better.