This is incredibly important. Rescuing people is very inefficient. It means people having to use AP's that will often take them over depleted zones early in the game. While this can allow them to pick up dropped resources, it's still far from ideal. The density and distribution of zombies is _random_. There are times when there are tons of zombies right up against one side of the town, or in a promising patch of foraging. Whenever you are planning to explore new spaces on the map, have an expedition of at least 3 people (and if you have so few, having some weapons is a good idea because 8-10 zombie packs are not unheard of and it is more complicated rescuing three people than one sometimes because people may not all be logged on when help arrives, see below). So, instead of rushing out as soon as your AP's are refreshed and you're sure they won't be needed for building, go to create an expedition. This makes a route on the map describing exactly what squares you wish to pass through and how you plan to get back to town. When you're using your map, select an expedition and the glowing dot that shows up by the direction arrows that normally points toward town will now point toward the next expedition square to enter (or at least in it's direction if you're off the expedition trail). Then post in forum. If people think your plan is good, they'll offer to join. If they don't, see if anyone else has an expedition worth joining.
The key thing is to move in packs of three to five people. It's not just about safety. This has a very important side effect that you will fully or nearly fully search each zone as you come to it. Searching is a zero AP action. So depleting the zone without having to waste 2 hours+ (or as few autosearches as possible) is a good thing. Just remember to mark the map properly. Because the map system only allows one detail to be marked, it is worth keeping a list in forum of depleted squares, exact zombie counts and dropped items and other important information.
Before starting out, it's worthwhile to have everyone in the group confirm four things:
(a) does everyone have the AP to go the full length of the expedition (AP both remaining and available through food, water, and in desperation, drugs (see above about why not to make drugs Plan A))
(b) has everyone dumped everything but weapons and AP sources back in the bank (while holding resources to prevent watchtower completion is important this is best delegated to people with zero AP left who are in town)
(c) If travelling into unknown territory, everyone should say what times they can be available, perhaps trading IM's (make an anonymous one just for the game if you're shy about privacy) but at a minimum committing to be on during the time windows they say they will be. This is critical because if the expedition is to be rescued, everyone rescued must leave the square within 30 minutes.
If the expedition is to resume moving, everyone should be there, anyway. But even if you have to abort the expedition it's important to abort it cleanly so only one resuce party is needed. If you have multiple unknown spaces to continue into after needing rescue, if you're planning to go on, then ask the rescuers if they will join you for some or all of the route through those unknown spaces.
(d) Make sure to the maximum degree currently possible that you're aware of zombie group sizes on the route and that you can have control in every square. Using water to kill zombies should be a last resort. Water equals AP's and is far better used to empower citzens than kill zombies. Be cautious of going through zones you barely have control of, if you have a choice (if you don't, don't sweat it) because accidents do happen and people get forced to leave the computer at bad times.