Yes, you can add as many users as you need to your account. To add a user, log into your account and at the top of the screen where you see your name, click on that and then click on Account Info. From there you will see the existing users on your account and you can add more by clicking on the Add User button.
When adding users you can provide them with specific roles so that only the users you want to make actions in your account can. For example, to add a moderator to your account, but restrict them from being able to buy additional walls or features, you'd pick the Limited Access option, and then set the Access Level to Moderator. Finally, you'd select the specific walls that you want them to have access to.
When adding additional users for moderation, all posts will be reserved to the individual users, and no two moderators will see the same posts. If you do not add moderators as their own users, but instead share your username/password, all users will see the exact same posts to moderate and you will not be able to efficiently moderate your wall.
If you create a wall after you've added a Limited Access user, they will not see the new wall. You'll need to go back to this page and edit them to have access to the newly created wall.