Something is intersectional if it takes different parts of a person's identity into account, including things like gender, age, religion, and race.
When a group is intersectional, it's making an effort to consider the fact that we all have many different identities — and some of them can make us vulnerable to being treated unfairly or misunderstood. Your school club's intersectional approach means that students with disabilities feel welcome there, along with those who have different religious beliefs or ethnic backgrounds. The intersection, or crossing, of these different identities is where intersectional comes from.