You must specify the header using the -H
option as noted in Bertrand Martel's answer. That will retrieve up to 20 users.
Above 20 users, you must get fancier. The JSON output is paginated, and each query is limited to 100 users per page. So to get 300 users, you must get three pages, 100 users at a time:
curl -H "Private-Token: kfjakjfkjkd" "https://company.domain.com/api/v4/users?active=true&per_page=100&page=1" > gitlabusers1.json
curl -H "Private-Token: kfjakjfkjkd" "https://company.domain.com/api/v4/users?active=true&per_page=100&page=2" > gitlabusers2.json
curl -H "Private-Token: kfjakjfkjkd" "https://company.domain.com/api/v4/users?active=true&per_page=100&page=3" > gitlabusers3.json