create and apply retention policy (to retain data not delete) for exchange mailbox to a group?

As mentioned in the documentation: https://learn.microsoft.com/en-us/purview/retention-settings#configuration-information-for-exchange-mailboxes-and-exchange-public-folders

If you use a static policy scope and choose recipients to include or exclude, you can select distribution groups and email-enabled security groups as an efficient way to select multiple recipients instead of selecting them one-by-one. When you use this option, behind the scenes, these groups are automatically expanded at the time of configuration to select the mailboxes of the users in the group. If the membership of those groups later change, your existing retention policy isn't automatically updated, unlike adaptive policy scopes.