Function to get which distribution groups someone is a member of.

Get-MemberOfDistributionGroup.ps1

function Get-MemberOfDistributionGroups
{
    [CmdletBinding()]
    Param(
        [String]$Identity
    )
    $user = Get-Mailbox $Identity
    "$($user.DisplayName) is a member of the following groups:"
    foreach ($Group in (Get-Recipient -Filter "Members -eq '$($user.DistinguishedName)'"))
    {
        "  -$($Group.DisplayName)"   
    }
}