You can use an action to get the members of an Azure AD group or a SharePoint online group.
Azure Active Directory - Get members of an Azure Active Directory group help ( https://help.nintex.com/en-US/nwc/Content/Designer/Actions/AzureAD-GetMembersofADGroup.htm )
In the output for these actions is a collection of just email addresses. You should be able to use that in the Assign a Task to Multiple users assignees.