Inputing into Cell creates Command button (or alternative) on userform

Instead of creating command buttons, why don't you have your user form read the sheet, fill in a two column list box and put a star next to the signed in users.

Dim DataRange as Range
Dim oneCell As Range

Set DataRange = Range(Range("A2"), Cells(Rows.Count,1).End(xlup))

With ListBox1
    .ColumnCount = 3
    For Each oneCell in DataRange
        .AddItem oneCell.Value & " " & oneCell.Offset(0,1).Value
        If oneCell.Offset(0, 2) <> vbNullString then .List(.ListCoun-1, 1) = "*"
    Next oneCell
End With