We've been going back and forth internally over the idea of XP sharing. So far, we've landed on *not* allowing this behavior because of various complexities involved in calculating this. That doesn't mean we won't change our mind, but, for now, I think it's more important to have users be able to show their group affiliations, and maybe their contributions to that group, at a glance so you can see how much they've contributed to a group's experience.
However, I understand the catch 22 here: if a new user joins a group and *only* contributes to the group, they'll be forever stuck at level 1. It's something we've been thinking about but haven't landed on a good solution for.
