How to add members and owners in private channels via script in bulk?

Use the Add-TeamChannelUser cmdlet, or the corresponding Graph API endpoint: https://learn.microsoft.com/en-us/powershell/module/teams/add-teamchanneluser?view=teams-ps
Here's a quick example on how to bulk add via CSV:

Import-Csv "blabla.csv” | % {Add-TeamChannelUser -GroupId GUID -DisplayName "channel name" -User $_.UPN}  

where the CSV has a column called "UPN" to designate the users