You can add Board Members only from edit Company form, under Board Members tab.
Fig. 1:Board Members Form
The Member Name is mandatory it can be a Company or a PersonContact. Also you should fill into the other mandatory fields: Designation Date and Role.
Otherwise you can edit or delete any member from the "Action Wheel" in the Board Members grid.
On the other hand, you can use the Designation Date and Until Date filters in order to filter your Board Members list view.
Fig.2 : Board Members History Report