You can group customers projects using the same currency into one invoice. The easiest way to create a grouped invoice is through Invoices > Create invoices. Choose all projects you want to group on one invoice and select Group projects on invoice before creating the invoice. Note that you can use the filters or search to limit the list of projects.
After creating the invoice you can review and edit projects in Detailed information section. If you remove a project, billing rows will be removed from the invoice and returned as billable. If you add new projects, billable rows are not automatically added as invoice rows but you can add them manually from Uninvoiced items section selecting all the rows you want to invoice