Use the Google Cloud console's IAM & Admin / IAM page to +ADD a new user with the below Roles (permissions).
NOTE: More narrow permissions would be more secure, but it'd take time and experiments to figure out what's really needed. E.g. Logging / Logs Viewer would probably be sufficient for debugging use, but then the person would need additional permissions to clean up old logs and maybe do some configuration.
User permissions/roles:
- Basic / Editor
- Cloud IAP / IAP-secured Tunnel User
- Logging / Logging Admin
Admin user additional permissions:
- Cloud IAP / IAP Policy Admin
- Cloud IAP / IAP Settings Admin
- Owner