Example of managing a Github organization with Terraform

github.tf

###
## <1> The two properties that need to be set are `token` and `organization`. You can hardcode them right in
##     this provider block (very unrecommended) but the better option is to set the `GITHUB_TOKEN` 
##     and `GITHUB_ORGANIZATION` environment variables. Terraform will see those and configure the provider with them.
###
provider "github" { }

###
## <2> This assigns a user to the organization. the `username` property should be their string username.
###
resource "github_membership" "test_user_membership" {
    username = "TestUser"
    role = "member"
}

###
## <3> This will create and manage a new team within the organization.
###
resource "github_team" "test_team" {
    name = "test-team"
    description = "A new team created with Terraform."
}

###
## <4> Assigns a user to a team. You can interpolate the team's id with the syntax below. 
##     For the `username` you again use the actual string username for the user.
###
resource "github_team_membership" "test_user_test_team" {
    team_id = "${github_team.test_team.id}"
    username = "TestUser"
    role = "member"
}

###
## <5> Assigns an existing Github repository to the created team. For `repository`, use the actual string name
##     of the repository.
###
resource "github_team_repository" "test_team_test_repo" {
    team_id = "${github_team.test_team.id}"
    repository = "existing-repo-name"
    permission = "pull"
}