You can do it like this:
[reflection.assembly]::LoadWithPartialName("System.DirectoryServices.AccountManagement")
$username = "samaccountname"
$ct = [System.DirectoryServices.AccountManagement.ContextType]::Domain
$user = [System.DirectoryServices.AccountManagement.UserPrincipal]::FindByIdentity($ct, $username)
$g = $user.GetGroups()
( $g | select -expa name ) -contains 'groupname'