In this post, I will explain how to programmatically connect an existing top-level SharePoint Online site to a new Microsoft 365 Group using PowerShell, using the Set-SPOSiteOffice365Group PowerShell command. Before proceeding, install the SharePoint Online PowerShell module and run the following commands:

# Connect to SharePoint Service
Connect-SPOService -Url "https://<yourtenant>-admin.sharepoint.com"

This example creates a new Microsoft 365 Group named “Group1” and connects the site collection to it. The group will have its privacy set to “Private”

$siteUrl = "https://<yourtenant>.sharepoint.com/sites/site1" 
$groupName = "group1"
$mailNickname = "group1"
Set-SPOSiteOffice365Group -Site $siteUrl  -DisplayName $groupName -Alias $mailNickname

Thank you for reading
/Simon