As long as a candidate is still in 'active' status, he or she will be able to submit another expense report using their same login and password. There is nothing that you need to do. If the candidate is 'inactive' then go into their profile and switch the status to 'active'.
You can also send a reminder to a candidate to submit their next expense report in Reimbi. Just open the candidate's profile and click the "send instruction email" link. This only displays for candidates that have a previously 'paid' expense report and are 'active'.