This command will add a new multisite with slug demo-X and a editor role user and output site URL, user name and user pass. You must create manually a site with slug "demo-1" before running the script first time

wp-cli-new-multiste.bash

wp site list --allow-root > sites-list.txt 
siteslist=`tail -1  sites-list.txt | head -1`
lastsiteurl=`echo $siteslist | egrep -o 'https?://[^ ]+'`
lasturlclean=`echo ${lastsiteurl::-1}`
lastid=`echo $lasturlclean|grep -Eo '[0-9]+$'`
((lastid++))
newslug="demo-${lastid}"
newsiteurl=`wp site create --slug=$newslug --private  --allow-root|awk '{print $5}'`
username="demo${lastid}"
useremail="demo${lastid}@demo.com"
timestamp=$( date +%T )
userpass="demo${lastid}${timestamp}"
wp user create $username $useremail --role="editor" --url=$newsiteurl --user_pass=$userpass  --allow-root
wp user list --url=$newsiteurl  --allow-root
message="Login URL: ${newsiteurl}wp-admin username: ${username} userpass: ${userpass}"
echo $message
echo "All Done"