wrote...
....
What I do is simply shade/fill cells when people are free in
GREEN ...and when one person is busy I shade them GREEN
(a bit tedious itself as I have to select each cell, then
choose the fill colour)

Right. Tedious. So don't use this approach.

If you have time slots in B1:U1 (every 1/2 hour from 8:00 AM to 6:00
PM) and names in A2:A49 (48 people), then select B2:U49, press [Ctrl]
+1, select the Number tab in the Format Cells dialog, choose the
Custom category, enter ;;; in the box just below Type: on the right,
and Click the OK button.

Next, with the same cells selected, run the menu command Format
Conditional Formatting, leave Cell Value Is in the rightmost entry
box, change the middle entry box to 'not equal to' using its drop-down
list, enter ="" (an equal sign followed by 2 double quotes, NOTHING
ELSE) in the right entry box, click on the format button use the
abbreviated Format Cells dialog to set the background color to green,
click OK in the Format Cells dialog, then when you return to the
Conditional Formatting dialog click OK there as well.

Now all you need to do is enter anything in any of the cells in B2:U49
to indicate that a particular person is available at a particular time
- much easier than formatting each cell.

You could also use Conditional Formatting in the time heading cells in
B1:U1 to check whether all or all but one persons are available, e.g.,
for 9:00 AM to 9:30 AM (col D), cell D1's conditional format would
have Formula Is in the leftmost entry box and the formula

=COUNTBLANK(B$2:B$49)<2

in the rightmost (only other) entry box. The only other thing needed
would be to give it a distinctive format.