As per my comment maybe try the following:
PowerShell ps = PowerShell.Create();
PSCommand cmd1 = new PSCommand();
cmd1.AddCommand("Import-Module");
cmd1.AddParameter("activedirectory");
ps.Commands = cmd1;
ps.Invoke();
cmd1.AddCommand("Add-PSSnapin");
cmd1.AddParameter("Name", "Quest.ActiveRoles.ADManagement");
ps.Commands = cmd1;
ps.Invoke();
cmd1.AddCommand("Get-QADMemberOf");
cmd1.AddParameter("identity", "rpimentel");
cmd1.AddCommand("where-object");
ScriptBlock filter = ScriptBlock.Create("$_.Name -ne 'Domain Users'");
cmd1.AddParameter("FilterScript", filter);
cmd1.AddCommand("Add-QADGroupmember");
cmd1.AddParameter("Member", "ktest");
ps.Commands = cmd1;
ps.Invoke();