You cannot give someone else the ability to share - it is a feature in our product backlog, but not one that we are currently working on implementing.

You can add email addresses to your account in the General Settings page, which will make that email also an owner of the account. The passwords for all emails will be the same, however, and everyone who you add will exactly the same level of access that you do - to whit, full access to all features.

Might be a workaround for now :)