Get the group's shareable invite link (cached once known). Anyone with the link can join — immediately when joinApprovalMode is off, or after approval when it's on.

Security:

• The organisationId in the URL must match the organisation your JWT was issued for — requests for a different organisation fail with 401.
• Requires CRM_ADMIN, ACCOUNT_OWNER, or CRM_BOT role; other roles receive 403.
• Groups are only supported on WHATSAPP_OFFICIAL channels whose number is an Official Business Account (OBA); ineligible numbers receive 422.