Why does this code work outside of this function but not as a function?

If I run the code below outside of the function wrapper, it returns the expected values, but when I run them inside the function, $confirmed and $missing end up null:

Function Check-groups{

    param($uGroups,$kGroups)
    $confirmed = @()
    $missing = @()
    foreach($t in $uGroups){
        if($kGroups -contains $t){
            $confirmed += $t
        }else{}
    }
    
    Echo $confirmed
    foreach($t in $kGroups){
        if($confirmed -contains $t){}else{$missing += $t}
    }
    Echo "this is missing"
    Echo $missing
    Write-Host $user.name" does not have the following groups:"
    Write-Host $missing
}

Why is this? How can I make the function work.

If you want to access the values contained in $missing and $confirmed then you have to return them as output from your function:

function Check-Groups {

    param(
      [string[]]$uGroups,
      [string[]]$kGroups
    )
    
    $confirmed = @()
    $missing = @()
    foreach($t in $uGroups){
        if($kGroups -contains $t){
            $confirmed += $t
        }
    }

    foreach($t in $kGroups){
        if($confirmed -notcontains $t){
            $missing += $t
        }
    }

    # these messages will be written straight to the screen buffer
    Write-Host "These groups are missing:"
    $missing |ForEach-Object { Write-Host " - '${_}'" }
    Write-Host "Total: $($missing.Count) group$(if($missing.Count -ne 1){'s'})"

    # the value after the return statement will be passed back to the caller as output
    return [pscustomobject]@{
        Confirmed = $confirmed
        Missing = $missing
    }
}

Now you just need to assign the output from the function to some local variable at the callsite:

PS ~> $userGroups = 'Finance','AllUsers','FullTimeEmployees'
PS ~> $kGroups = 'AllUsers','FullTimeEmployees','MailUsers'
PS ~> $groupsAnalyzed = Check-Groups -uGroups $userGroups -kGroups $kGroups
These groups are missing:
 - 'MailUsers'
Total: 1 group
PS ~> $groupsAnalyzed.Missing
MailUsers
PS ~> $groupsAnalyzed.Confirmed
AllUsers
FullTimeEmployees
PS ~>