Companies Act_Practical Problems_15

In the Board meeting, where quorum was of three Directors, four Directors were present at the meeting when it began. All participated in the discussions (audio recording is available) but only two signed the attendance register and other two refused to sign. Does this invalidate the meeting?

Is it necessary that quorum should be present at the time of each & every agenda being discussed at the meeting? OR Is it sufficient if it is present only at the beginning at the meeting? Whether the answer is same for Board and General Meetings?