I want to set all user mailboxes that are members of a security group and do not have activesync to have it enabled.

$m = Get-Recipient -resultsize unlimited -Filter {(RecipientTypeDetails -eq 'UserMailbox') -and (MemberOfGroup -contains 'security group')}
foreach ($mailbox in $m){
    Get-CASMailbox $mailbox | fl activesync*
    if ((Get-CASMailbox $mailbox).ActiveSyncEnabled -eq $false){
        Set-CASMailbox $mailbox -ActiveSyncEnabled $true
    }
}