Here's a simple yet useful method for checking membership in a SharePoint group. This method will return true or false to indicate if the current user is a member of the specified SharePoint group in the current site. The current user is obtained from an assumed impersonation context.

public static bool IsUserInGroup(string groupName)
{
    bool inGroup = false;

    try
    {
        SPGroup group = SPContext.Current.Web.Groups[groupName];               
        inGroup = group.ContainsCurrentUser;
    }
    catch (Exception ex)
    {
        // SharePoint throws an exception if the group indexer
        // is not a group the current user belongs to.   
        inGroup = false;
    }
    return inGroup;
}