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group12 = user 1 and user 2
group13 = user 1 and user 3



cd /var/sftp/uploads
mkdir Folder1 Folder2
chown root:group12 Folder1
chown root:group13 Folder2
chmod 770 Folder1 Folder2

Create a group with users 1 and 2 as members, and another group with users 1 and 3 as members. Assign those groups ownership of the appropriate directories and grant them full privileges.