Problem Definition

Currently Jira Cloud Migration Assistant for Cloud to Cloud migration do not have option to preserve group membership.
https://support.atlassian.com/migration/docs/perform-a-cloud-to-cloud-migration-for-jira/
https://support.atlassian.com/migration/docs/how-cloud-to-cloud-migration-manages-users-and-groups/

Suggested Solution

Provide option to preserve group membership while migrating data between cloud sites using Jira cloud migration assistant.

Workaround

1. Export all users along with group membership, products from (https://admin.atlassian.com > Users > Export users > All users, groups, products)
2. Use rest API https://developer.atlassian.com/cloud/jira/platform/rest/v3/api-group-groups/#api-rest-api-3-group-user-post to add the users to the specific group
3. Provide product access to the specific group manually on cloud.