Select Parameters With Gridview

I have a form that has 4 fields to fill in. I have a button that can add these fields to a sql database. I also want to put a button next to the "add" button so you can fill out the same fields and search for those values. Here's what i have so far. I want to select based on the fields, but i'm having trouble with the syntax of the parameters. I also want to add something, so if nothing is filled out in one of those boxes, it retuns back all records for the default value.

  string strConnection = ConfigurationManager.ConnectionStrings["TimeAccountingConnectionString"].ConnectionString;
SqlConnection myConnection = new SqlConnection(strConnection);

String selectCmd = "SELECT * FROM users WHERE firstname = @firstame or lastname = @lastname or office = @office or team = @team";
SqlDataAdapter myCommand = new SqlDataAdapter(selectCmd, myConnection);

myCommand.SelectCommand.Parameters.Add(new SqlParameter("@firstname", SqlDbType.VarChar, 50));

myCommand.SelectCommand.Parameters.Add("@firstname", txtFirstName.Text);
myCommand.Parameters.AddWithValue("@lastname", txtLastName.Text);
myCommand.Parameters.AddWithValue("@team", dwnTeam.Text);
myCommand.Parameters.AddWithValue("@office", dwnOffice.Text);

DataSet ds = new DataSet();
myCommand.Fill(ds, "users");

MyDataGrid.DataSource = ds.Tables["users"].DefaultView;
MyDataGrid.DataBind();