Ok you have to add the person you want to ride with to your group. Then make sure they're not on a mount of there own. After all of that an interaction symbol thing should appear. Your friend presses x and then your riding together.
Hope his helps, Courtney Greenthistle level 61 thaumaturge
OK to answer that question madgurl135 your are completely wrong. In order for a person to ride the rhinoceros mount. The person who owns the mount must add the person to a group once that is established the person can press X and they will be able to ride on the mount. Note the mount will not automatically put you back on the mount once you enter and indoor place then leave. You will have to find person and hop on again in order to ride again.