In the centralized user management, a user access admin is allowed to grant product access within their scope via the admin hub.
Utilizing the groups API (ie. from Jira), they are not allowed to perform group membership changes and so they will not be able to perform bulk operations.

curl --request POST \
  --url 'https://site.atlassian.net/rest/api/2/group/user?groupId=PRODUCT_ACCESS_GROUP' \
  --user 'USER_ACCESS_ADMIN ' \
  --header 'Accept: application/json' \
  --header 'Content-Type: application/json' \
  --data '{
  "accountId": "ATLASSIAN_ACCOUNT_ID"
}'
{"errorMessages":["You are not authorized to perform this action. Administrator privileges are required."],"errors":{}}%     

Suggestion

Allow an option for User Access Admins to perform bulk operations via the Admin hub or programmatically via API or via the UI