Automatically following (star) all SharePoint Sites for users, based on https://www.cyberdrain.com/automating-with-powershell-automatically-following-all-sharepoint-sites-or-teas-for-all-users/

Autostar.ps1

$TenantID = 'CUSTOMERTENANT.ONMICROSOFT.COM'
$ApplicationId = "YOURPPLICTIONID"
$ApplicationSecret = "YOURAPPLICATIONSECRET"

$body = @{
    'resource'      = 'https://graph.microsoft.com'
    'client_id'     = $ApplicationId
    'client_secret' = $ApplicationSecret
    'grant_type'    = "client_credentials"
    'scope'         = "openid"
}

# Target users in a specific group only
$groupId = "xxxxx-xxx-xxx-xxx-xxxxx"

$ClientToken = Invoke-RestMethod -Method post -Uri "https://login.microsoftonline.com/$($tenantid)/oauth2/token" -Body $body -ErrorAction Stop
$headers = @{ "Authorization" = "Bearer $($ClientToken.access_token)" }

# Retrieve the list of users who are members of the specified group
$users = @()
$nextLink = "https://graph.microsoft.com/beta/groups/$($groupId)/members"
while ($nextLink) {
    $result = Invoke-RestMethod -Uri $nextLink -Headers $Headers -Method Get -ContentType "application/json"
    $users += $result.value.id
    $nextLink = $result.'@odata.nextLink'
}

# # Debug specific users
# $users = @(
#     "xxxxx-xxxx-xxx-xxx-xx",
#     "xxxxx-xxxx-xxx-xxx-xx"
# )

$users | Measure-Object

# SharePoint Site IDs
$site1 = "xxxx.sharepoint.com,xxxxx-xxxx-xxx-xx-xx,xx-xx-xx-xx-x"
$site2 = "xxxx.sharepoint.com,xxxxx-xxxx-xxx-xx-xx,xx-xx-xx-xx-x"

foreach ($userId in $users) {
    # write-host $userId

    $AddSitesbody = [PSCustomObject]@{
        value = [array]@(
            @{
                "id" = $site1
            },
            @{
                "id" = $site2
            }
        )
    } | ConvertTo-Json

    $FavoritedSites = Invoke-RestMethod -Uri "https://graph.microsoft.com/beta/users/$($userid)/followedSites/add" -Headers $Headers -Method POST -body $AddSitesBody -ContentType "application/json"
    # $FavoritedSites | Measure-Object    
    # write-host $FavoritedSites
    write-host "Added sites to favourites for $userId"
}