GNU Screen multiuser mode allows for separate focus, but only one cursor active in a given tab:
https://www.gnu.org/software/screen/manual/html_node/Multius...
It allows for multiple viewers of a single tab, but not mutiple cursors like Zellij has.
GNU Screen does allow multiple users editing in separate tabs in a single session. It was quite useful for devops code reviews of emergency configuration changes on remote servers.
I look forward to trying out Zellij.