The user doesn't exist in the tenantIssue Error: "the user doesn't exist in the tenant"Unable to allow outside users to join conference calls through Microsoft teams External users cannot join a conference call via MS teams Guest users are denied access to join a conference call in Teams IntegrationCauseThe error which the user is seeing is expected. Multitenancy is an architecture where multiple tenants share the same physical instance of the app. Although tenants share physical resources (such as VMs or storage), each tenant gets its own logical instance of the app.ResolutionWe cannot add the users from other tenants directly, However, we can add users from other tenants as guest users. To do so, the user needs to create a user as a guest in the tenant directory. Below is the doc that details this process. After creating the guest user, he would be able to add them either by using the email from the start chat module or creating a sys_record for the guest user. https://docs.microsoft.com/en-us/azure/active-directory/external-identities/b2b-quickstart-add-guest-users-portal Research: A brief explanation of multi-tenant architecture can be found in this link (https://docs.microsoft.com/en-us/azure/architecture/multitenant-identity/#:~:text=Multitenancy%20is%20an%20architecture%20where,but%20not%20with%20other%20tenants.)