Import-Module ActiveDirectory
$groupname = PseudoDynamicGroup
$users = Get-ADUser -Filter * -SearchBase "ou=desiredUsers,dc=domain,dc=tld"
foreach($user in $users)
{
  Add-ADGroupMember -Identity $groupname -Member $user.samaccountname -ErrorAction SilentlyContinue
}
$members = Get-ADGroupMember -Identity $groupname
foreach($member in $members)
{
  if($member.distinguishedname -notlike "*ou=desiredUsers,dc=domain,dc=tld*")
  {
    Remove-ADGroupMember -Identity $groupname -Member $member.samaccountname
  }
}

import-module ActiveDirectory
Get-ADGroupMember OU_GroupName | % { Remove-ADGroupMember 'OU_GroupName' -Members $_ -Confirm:$false}
Get-ADUser -SearchBase 'OU=OUName,OU=ParentOUName,DC=DomainName,DC=TopDomainName' -Searchscope 1 -Filter * | % { Add-ADGroupMember 'OU_GroupName' -Members $_ }
Add-ADGroupMember -Identity "OU_ParentName" -Members "OU_ChildOneName", "OU_ChildTwoName", "OU_ChildThreeName"

Function AddUserToGroup($Group, $User)
{
 if(!(Get-ADGroupMember -Identity $group | ?{$_.name -eq $User}))
 {
  Add-ADGroupMember -Identity $group -Members $User -Server $DomainController
 }
 else
 {
  Write-Output "The user: $User is already in the $group"
 }
}

Function RemoveUserFromGroup($Group, $User)
{
 if(!(Get-ADGroupMember -Identity $group | ?{$_.name -eq $User}))
 {
  Add-ADGroupMember -Identity $group -Members $User -Server $DomainController
 }
 else
 {
  Write-Output "The user: $User is already in the $group"
 }
}