Some groups require you to meet certain criteria to be accepted as a member and/or require you to submit a short application. If your application is rejected, you probably do not meet the requirements set by the group leader.
Thematically, it makes sense for some groups to limit membership to people who meet certain criteria (e.g., certain age and/or gender). In addition, it is up to the group leaders whether they want to accept members who have not yet participated in the free authenticity check. You should therefore find out in advance what requirements you have to meet for the group in question.
Basically, the group leader is required to give you reasons why your application was rejected. If they fail to do so, you are of course free to ask for the exact reason.