To invite people to your image bank, you need to be an administrator or the owner of the image bank.
If you've been assigned either of these roles, you invite people by going to "... More" in the top left corner and then to "Invite people".
Enter the email of the user and click enter. The user will default to have the "user" rights of the image bank. Hover the mouse over the email and you are given the option to make the user an administrator instead.
Once you click on "Save", the user will receive an invitation email.
Under "Manage Members" you can get an overview of the members of your image bank and change member's roles.