Your organization treats people in a very "exclusionary" manner. ( Meaning that it is a group that is not accepting of others not of themselves.
There is 'cliquey':
Cliquey, the adjectival form of Clique:
1 - a small, exclusive group of people; coterie; set.
A group that closes themself off from outside influences will often be called "insular"
characteristic of an isolated people; especially : being, having, or reflecting a narrow provincial viewpoint
Though this might sound ill fitting, their examples show how it fits your needs:
Examples of insular
the insular world of boarding schools
an insular community that is not receptive of new ideas, especially from outsiders
The best choice is discriminatory: Marked by or showing prejudice, bias.
Your organization treats people in a very discriminatory manner.