Adding alternative host to a webinar.

Hello @HarryT 

Co-hosts can be added during a webinar, but not prior. If there are users on your account, you can make them alternative hosts which will make them co-hosts once you join into the webinar as host. The following articles can assist in the roles in your webinar, adding an alternative host, as well as adding a co-host:

https://support.zoom.us/hc/en-us/articles/360000252726-Roles-in-a-webinar

https://support.zoom.us/hc/en-us/articles/206330935-Enabling-and-Adding-a-Co-Host

https://support.zoom.us/hc/en-us/articles/208220166-Alternative-Host

I hope this helps and please make sure to mark the solution as accepted if this information is what you needed.