Hi @Lisa Freeman ,
First, make sure external sharing is enabled.
https://learn.microsoft.com/en-us/sharepoint/turn-external-sharing-on-or-off
https://learn.microsoft.com/en-us/sharepoint/change-external-sharing-site
Then You can ask the site owner to grant him the appropriate permissions directly at the site level.
Click the small gear in the upper right corner of SharePoint----Site permissions----Advanced permission settings to enter the following page:
Then click Grant Permissions and enter his account and the permissions you want to grant it in the pop-up window. Click share and the other party will receive the email.
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