How to create custom grid

There's no exact solution for this available in base Appian.  My first approach would be to build a Read-Only Grid where the Name and Code column values are Rich Text, and iterate over all the values that belong in that primary row, separating them with line returns.  There are other possible "approximate" approaches too, like perhaps building a quasi-grid using stacked Cards, etc.

Here's a "portable" example of my primary suggestion above, using your example data structure:

a!localVariables(
  local!array: {
    a!map(
      date: today() - 5,
      people: {
        a!map(name: "Dave", code: 1122),
        a!map(name: "Frank", code: 1133)
      }
    ),
    a!map(
      date: today() - 4,
      people: {
        a!map(name: "Maria", code: 3322),
        a!map(name: "John", code: 1324),
        a!map(name: "Joe", code: 3444)
      }
    ),
    a!map(
      date: today() - 3,
      people: { a!map(name: "Adam", code: 3432) }
    )
  },
  a!gridField(
    data: local!array,
    spacing: "DENSE",
    columns: {
      a!gridColumn(
        label: "Date",
        sortField: "date",
        value: fv!row.date
      ),
      a!gridColumn(
        label: "Name",
        value: a!richTextDisplayField(
          value: {
            a!forEach(
              fv!row.people,
              fv!item.name & if(fv!isLast, "", char(10))
            )
          }
        )
      ),
      a!gridColumn(
        label: "Code",
        value: a!richTextDisplayField(
          value: {
            a!forEach(
              fv!row.people,
              fv!item.code & if(fv!isLast, "", char(10))
            )
          }
        )
      )
    }
  )
)