Help with merging personal story with friend
You only get experience and rewarded at your own story line. If you and your friend are on the same storyline quest, you will both get rewarded. I believe if you are not on the same story line, you will not get exp or a reward. The “join” feature just happens whenever you are in the same party and in the same area, no matter what quest you are on or level difference. Hope this help :P
Based on this page http://wiki.guildwars2.com/wiki/Personal_storyline it looks like your personal stories will merge around level 29, when you choose an Order to join. If you made some similar choices in character creation, it may merge before that.
If you are on the same story step, with the same story line, you will have the option of accepting your friend’s decision and getting credit for your own personal story step. My friend and I both have Asuras and I’m pretty sure our stories merged before 30, but I can’t remember when.