What is the difference between "external_id" and "group_id"?

The "group_id" is an internal ID that is automatically created by Flip.

When creating a group via the API, the "external_id", which is assigned by the client, is the unique ID.

In order to make a manually created group editable via the API and to be able to add users to this group, the "group_id" must first be queried via the "Get all existing groups" request. Then you have to give this group an "external_id" and the group name ("name") via the "Create or update multiple users at once" request.