How to reserve seats online, in advance with JR Pass

Hello :), my partner and I are wanting to get the JR Pass for our travel at the end of march since we will be going to multiple cities. The group we are going with is only going to one city so they will not be buying the pass but we are traveling with them to Osaka and want to be on the same train. How do I reserve seats online once I have purchased the pass? I have not purchased the passes yet for fear we won't be able to travel together! Please help!