public static Map<String, String> listGroups() {
HTTPRequest req = new HttpRequest();
req.setMethod('GET');
req.setHeader('Accept', 'application/json');
req.setEndpoint('callout:PrimaryEndpoint/');
HTTPResponse res = new Http().send(req);
....
}

public static Map<String, String> listUsers() {
HTTPRequest req = new HttpRequest();
req.setMethod('GET');
req.setHeader('Accept', 'application/json');
req.setEndpoint('callout:PrimaryEndpoint/users');
HTTPResponse res = new Http().send(req);
....
}

public static Map<String, String> listFiles() {
HTTPRequest req = new HttpRequest();
req.setMethod('GET');
req.setHeader('Accept', 'application/json');
req.setEndpoint('callout:PrimaryEndpoint/files');
HTTPResponse res = new Http().send(req);
....
}

public static HttpResponse doCallout(String urlPath) {

    HttpRequest req = new HttpRequest();
    req.setMethod('GET');
    req.setHeader('Accept', 'application/json');
    req.setEndpoint('callout:PrimaryEndpoint/' + urlPath);
    HttpResponse res = new Http().send(req);

    //do some error checking here
    return res;

}