Here’s how you add users to existing SIteGroups, –check Part 1 for PS script on how to create these Groups.

First create a UTF-8 .csv file with all users and groups
– Groups, LoginName must exist, and off course the SiteUrl must be correct.

Run this PowerShell script
PS>Import-Csv C:\admin\SMusersToGroups.csv | where {Add-SPOUser -Group $_.Group -LoginName $_.LoginName -Site $_.Site}

Tip: Just add more users to any group, and rerun the script, then they will be added to their group.