When you create manual invoices from approved time and expenses, you have an option to Group by item in the Create Invoice dialog. This option allows you to group entries with common activity or expense items as a single line item on the manual invoice. In other words, it combines multiple time and expense entries that have a common activity or expense item into one entry during the creation of the invoice.
For batch invoices, you cannot directly combine time and expense entries based on common activity or expense items. However, you can achieve a similar effect of combining the entries on invoices by customizing the invoice template in Settings > Billing & Invoices > Invoice Templates. Check CORE Help for details on that. In particular, you can modify the options under Layout > Time/Expense tab in the Edit Template panel to suit your requirements. For example, you can set Group By to Activity under Time tab and to Item under Expense tab to group the time and expenses by common activity and expense items on the invoice. Then you can set Time/Expense Details to Summarized under the Time and Expense tabs to display a single line item for each group on the invoice. This creates a similar effect as the manual invoice above. You can save the changes as a new template and apply it to your invoices in future. Check CORE Help Center for details on changing the invoice template.