API or NET client library to add users to an organization or projects under it?

Take this by using Azure DevOps REST API

To add users to an organization or project, use the User Entitlements API:

{
  "accessLevel": {
    "accountLicenseType": "express",
    "licensingSource": "account"
  },
  "user": {
    "principalName": "newuser@example.com",
    "subjectKind": "user"
  },
  "projectEntitlements": [
    {
      "group": {
        "groupType": "projectContributor"
      },
      "projectRef": {
        "id": "your-project-id"
      }
    }
  ]
}

You’ll need:

• Organization name
• Project ID
• User email
• Proper PAT (Personal Access Token) with admin permissions