Add people to a static uploaded segment

For school administrators using GiveCampus Outreach. Controls can differ between the classic email editor and the newer Composer editor.

The Upload a List flow creates a new static segment; it does not append CSV rows to an existing uploaded segment. Prepare a combined, deduplicated CSV containing existing and new members, upload it as a clearly named replacement segment, and switch intended audiences to that segment after reviewing membership. Preserve required external_id, first_name, last_name and email_address or phone_number headers. Existing outreach audiences do not automatically switch to a new segment.