Guarding in PUG scenarios and in a premade are totally different skillsets almost.
In a premade you have voice comms, proper group comps, and setup/strategy. So for instance in our 2-2-2 groups we used to run we had our most defensive tank guarding the slayer/chop, and then the other on mara/WL duty. Voice comms lets us instantly know when for instance the slayers tank is punted so the other tank can instantly switch. Even going the extra mile a really good tank player can tell if another tank is attempting to punt him and letting the other tank know might decrease the time it takes to switch by a second or 2. The other tank can also make sure he wont be inside a GCD when trying to switch guard.
Voice comms also means you can have plans for what do to when your healers are swamped (who is switching guard, and how thie DPS should act without guard etc...), when to get back and when your tanks are CCed and of course tanks can communicate with their gaurdee's when they are out of range or putting themselves in a bad position. Shot caller also means people aren't running aimlessly.
In a pug you are at the absolute mercy of RNG for your group comp, virtually no communication, and it tends to bring out the absolute worst habits in even good players.
The first thing you should do is look at groups and find the best MDPS you can, switch if you can and put guard on them as soon as you load in because good players are going to have unit frames that tells them when they have guard, having guard before they leave spawn is going to make them much more confident and play differently.
Second thing to do is look at your healing situation, how many healers do you have? You need to play completely differently when you have 1 or no healer than when you have the proper 2 in party. And of course make sure everyone is together before you push backlines, don't Rambo at fort when both your healers are still 10 seconds behind. And don't run into backlines when there is a guarded slayer and WL tearing ass on your healers.
But of course since you are guarding someone else you have to pray they are doing these things as well, if you are guarding some mara and he runs after some BW while your healers are swamped it's really a **** situation because you need to guard healers, cc the DPS on them, but the mara is just going to run into his death. PUG tanking really is all about triage in that manner.
Since MDPS is ****, and 100x more **** in pug play you are going to run into a lot of scenarios where you don't have MDPS to guard and that creates a weird dynamic where you have to choose to either lone wolf it in (while spamming your assist macro on RDPS don't be a noob and try to solo kill ****), or to play back with your RDPS/healer and help them survive via guard/CC but losing most of your lethal damage. Usually you have to do the later in a losing scenario, or a scenario where you are low on healing.
Lowbies also present a pretty big problem specifically when there is a good WE/WH on the other team. If your only healer is a lvl 33 shaman and you have a high RR WH on the other team just instagibbing him you need to just suck it up and play to keep that shaman alive and keep the WH cucked.
Other than lowbies you need to be able to recognize bad players, specifically MDPS players that aren't going to be worth guarding. This is based on who the are targeting (if you are really good you can quickly switch between all the DPS in sc and spam assist and see who is assisting each other and picking good targets), and how they are taking fights and positioning themselves. obviously if you are guarding some choppa who just make a B line backlines bumrush 100% of the time and dies while trying to kill some rdps who doesn't even have someone assisting with him you might as stop guarding him if there is someone else too. If he gets mad enough he will go PvE which is actually a pretty nice boon. If you are ever up when mieles is spamming for a guard in /5 join him and see how a mdps should play.
Outside of tanking it's really up to the DPS in the SC to assist each other, really the RDPS need to be slamming that MF assist button on MDPS, and RDPS when MDPS aren't in the ****.
tldr PUG tanking is probably the hardest thing to do properly because of the almost infinite variables you need to learn and take into account.
). But also have your eye on the back line, and if somebody starts nuking your healer/rdps, switch the guard and run towards the back line.
