Hi when databinding (to anything) you need to set the DataTextField and DataValueField of your DropDownList. In your case you should use the following code
List<ListItem> users = new List<ListItem>();
foreach (SubscriptionUser su in subscriptionDetails.UserList)
{
users.Add(new ListItem(su.FirstName + " " + su.LastName, su.EmailAddress));
}
ddlPrimaryContact.DataTextField = "Text";
ddlPrimaryContact.DataValueField = "Value";
ddlPrimaryContact.DataSource = users;
ddlPrimaryContact.DataBind();