At this time XP accounts can only have one login attached to them. Which means, if there's someone else on your team who needs to log in to your account, you'll need to share your login information with them.
Yes, we are planning to add the sub-account feature in the future-- it hasn't been a high priority for us simply because we're preferring to focus first on the features that are going to make you dramatically more money in your business.
Hope that's OK! ?