I have troubles with adding my application to group for any test user. I\'ve created two test users and one of them has group and he is an admin there and I wrapped those us
In order to test the Groups API with an app in development mode, you need to be an admin of both the app and group. Test users don't count as admins of the app. Therefore, you'll need to use a real user account. You don't have to add the app to the group, it is assumed that the app is added if you are the admin of the group.
Source: Facebook Platform Bug Report.
Additional information from another useful bug report:
to read group information (such as the /events edge on a group), the app needs to have the "Groups API" feature reviewed and approved, and the app needs to be installed to the group. No additional permissions are required on the access token.
to read information about the members of a group, the access token needs to have the groups_access_member_info permission granted during login, and the app must have been approved for the "Groups API" feature.
to publish to a group, the access token needs to have the publish_to_group permission granted, in addition to the app having the "Groups API" feature
When testing this with your own groups in development mode, the Groups API feature is assumed to be enabled, and when querying groups that you manage, the API will behave as though the app is installed to your group. Note that when querying group posts, you may only receive posts made by yourself or other admins of the app.
When querying your group's events with your app in development mode, the call worked as you didn't need any additional permissions on the access token, and the API behaves as though your app is installed on the group. This is similar to how this would work in production, as the access token wouldn't need any additional permissions to be granted during login.