Time Machine

The MacBookPro has a neat back up system "Time Machine".
This is quick and easy to use with a 3TB dedicated LaCie hard drive.
BUT I am not sure how to use this on our MBP which has 4 users - 2 main users and 2 infrequent users.

Should each user log on and initiate a TM Backup?

Or when any user logs on and initiates a TM Backup does that copy all changed files for all users?

Looking at the size of the backup files on the LaCie, it appears that each user must do his own thing.

Ignoring the 2 minor users and concentrating on the two major users A and B:

User A backup seems stable at around 75 GB.
User B seems to be around 88 GB.

Looking at the MBP hard drive it appears to have used 140 GB.

Does this make sense?

I am not too bothered about the 2 minor users, but I would like a secure back up for both user A and user B.

Any advice on how that is achieved?

Have a look at these FAQ, in particular point 26.

Your whole machine is backed up, unless you exclude bits.

Thanks to AEP and ian_c for those helpful answers.
Reassuring to know the system is looking after us.

The best thing that has happened to me in the whole of 2014!

Well, Time Machine got by rear end out of trouble just before Christmas when my old 2007 MBP developed hard drive issues that couldn't be repaired without a reformat.

Seemed a good time to treat my self to a new machine, since flakey discs rarely become unflakey (got an MBP 13" with an SSD).

From deciding a new machine was needed to being back up an running was probably 6 hours, including the time it took to go to Watford and buy the new kit. 3 accounts (admin, main user, diagnostic user) A few serials needed reentering but other than that it was flawless.

I have Dropbox as fallback for mission critical stuff.

And now the old one is a media server until it falls over completely.

I use both Time Machine and SuperDuper via a large external Drive - and yes both have got me out of trouble in the past - very impressively so too