Question: Is it possible to invite external guest members to open or moderated communities?

Answer: No, open and moderated communities are designed to be limited to members of your organization. For communities that require participation from guest members outside the organization, use restricted communities with the option to allow members from outside the organization enabled.

For the steps to create a community and description of the access levels (open, moderated, restricted), refer to the product documentation topic Creating communities

Note that you cannot change a community that does not allow guests to one that does. This behavior is noted in the warning that appears when you select the access level: "Once a community is designated as solely for your organization, it can never be changed to allow people from outside your organization."