How to display the tasks assigned to specific group in process task report?

Create a constant of the group and use that constant into ContextGroups.

a!queryProcessAnalytics(
  report: cons!OPEN_REIMBURSEMENT_REQUESTS,
  contextGroups: cons!EXPENSE_APPROVERS,
  query: a!query(
    pagingInfo: a!pagingInfo(startIndex: 1, batchSize: 20),
    logicalExpression: a!queryLogicalExpression(
      operator: "AND",
      filters: {
        a!queryFilter(field: "c2", operator: "includes", value: "East"),
        a!queryFilter(field: "c4", operator: ">", value: 10000)
      }
    )
  )
)

For more details: a!queryProcessAnalytics() Function - Appian 21.4

Adapt the below pattern to a group context report.

docs.appian.com/.../recipe-display-a-users-tasks-in-a-grid-with-task-links.html