can I have some informations abuot developer's activities? can I join or test an existing project or I have to open one?
Talk to herds / project that you're interested in joining and/or see http://www.gentoo.org/proj/en/devrel/staffing-needs/index.xml for areas needing help. You should probably also consider the Arch Tester projects that some archs are running. See http://www.gentoo.org/proj/en/base/x86/at.xml and http://www.gentoo.org/proj/en/base/amd64/tests/index.xml for details.