You don't need to use the Select-Object for this.
Just handle the IP addresses in a ForEach loop, and use the values in a string.
Something like this
$VMInfo=$null
$VMInfo+="<th><center>VM Name</th><th><center>IP Address</th>"
Get-VM-Namedb1-PipelineVariablevm|
ForEach-Object-Process {
$_.guest.IPAddress |
ForEach-Object-Process {
$VMInfo+="<th><center>$($vm.Name)</th><th><center>$($_)</th>"
}
}