When adding a user via Confluence or Bamboo it appears that in Bamboo that these users are not assigned to the groups you selected.

So when you add a user via Bamboo, you select the groups you want to be added too, but when you view the created user they are not in the groups you selected.

This is most likely due to the aggressive caching present in Crowd, this may need to be revised on add's and updates so the cache is more in-line with the current system state. The reason why I think this is the case is that it was reported after a certain point in time the user in question was 'suddenly' in the group