Or simply run this script in the browser console after loggin in to harbor (no manual copy of authorization/csrf headers neccesary):

await (async (user) => {
  let page = 0;
  let repos;
  do {
    page++;
    const reposResponse = await fetch(`https://harbor.akdb.de/api/v2.0/projects?page=${page}&page_size=100`);
    const csrfToken = reposResponse.headers.get('x-harbor-csrf-token');
    repos = (await reposResponse.json()).map(repo => repo.name).filter((repo) => !['library'].includes(repo));
    await Promise.all(repos.map((repo) => {
      return fetch(`https://harbor.akdb.de/api/v2.0/projects/${repo}/members`, {
        method: "POST",
        headers: {
          "Content-Type": "application/json",
          "accept": "application/json",
          "X-Harbor-CSRF-Token": csrfToken,
        },
        body: JSON.stringify({
          "role_id": 2,
          "member_user": {
            "username": user,
          },
        }),
      });
    }));
  } while (repos.length > 0);
})('m.mustermann');

This snippet can be used to copy members (and their roles) from one project to another. Run this script in the browser console after logging in to harbor:

const projectSource = 'ozg-meldewesen';
const projectTarget = 'ozg-finanzwesen';

await (async (projectSource, projectTarget) => {
    let page = 0;
    let members;
    do {
        page++;
        const membersResponse = await fetch(`https://harbor.akdb.de/api/v2.0/projects/${projectSource}/members?page=${page}&page_size=50`);
        const csrfToken = membersResponse.headers.get('x-harbor-csrf-token');
        members = (await membersResponse.json());
        await Promise.all(members.map((entity) => {
            return fetch(`https://harbor.akdb.de/api/v2.0/projects/${projectTarget}/members`, {
                method: "POST",
                headers: {
                    "Content-Type": "application/json",
                    "accept": "application/json",
                    "X-Harbor-CSRF-Token": csrfToken,
                },
                body: JSON.stringify({
                    "role_id": entity.role_id,
                    "member_user": {
                        "user_id": entity.entity_id,
                    }
                })
            });
        }));
    } while (members.length > 0);
})(projectSource, projectTarget);

The JavaScript snippets are now in confluence: https://digitale-verwaltung.atlassian.net/wiki/spaces/OZGDEV/pages/1876426930/Harbor+user+management