Private community administrators should be able to add users to the community manually, since users can't add themselves to communities that are private.
This could be seen as either a bug or an enhancement - from my perspective it seems that private communities are unusable without this change.
Here's our ideal setup:
Walled Garden with communities that are all private and each one has an administrator that controls which users gain access to those communities. We have a lot of small groups within our organization that we want to keep information from others (particularly contact information and photos). Without a way for the community administrators to manage the users of their community, it makes working with Elgg and impossibility.