Right now list sharing for work or school accounts is limited to members within your organization. If a member outside of your organization has invited you to join a list, then that list will be unavailable for you. Likewise, if you've invited someone who does not belong to your organization to join your list, that list will be unavailable for them.
As a workaround, we can recommend using personal Microsoft accounts for sharing outside of your organization. In the event that you need to share directly from your organization, such as sharing with a client, then you may be interested to know that we are considering adding guest access in the future.