You’re asking about Nature tests, but the answer applies to all tests. The key is what the manual calls Complex Obstacles (p.92), but I’ll go into more detail because you’re learning the game.
The usual way of doing things is that the GM presents the problem and includes the GM’s way of looking at it. So, for example, the GM says: “there’s a river you need to cross, building a bridge requires an Ob 3 Carpenter test, or you can just swim across, and that’d be an Ob 4 Health test”. The patrol can then present and argue for their own alternatives (like building the bridge using Stonemason instead).
The first option is likely a simple obstacle, with one mouse (probably the most skilled Carpenter) taking on the bulk of task with help from the patrol. If they succeed in building the bridge, the patrol can then cross the river together, problem solved. However, you could also resolve it as a complex obstacle if you want more narrative detail (and more chances for Advancement), testing in sequence: one test for gathering materials, another test to drawing plans, and finally a test to build the thing.
The second option should definitely be resolved as a Complex Obstacle in my opinion, with each mouse making the Health test to try and swim across the river. I’d consider those tests simultaneous instead of a sequence, and would therefore put some limits on Helper dice.
Now, you have to be careful with the consequences of these tests. It’s one thing if everyone makes it across because the failed tests passed with a Condition, and another if a failed test got a Twist and a member of the patrol went downstream and is now separated from the rest. If you’re not going to consider individual consequences (in this example, you might not want to risk splitting the party), you might aswell had resolved the problem as a simple obstacle, since the result was basically the same as with Helpers getting Conditions.
So, going back to your Nature example, if the patrol is trying to climb up a cliff, you could do something similar and test each mouse, but remember to carefully consider the possible consequences. If they are all tethered by a rope, one of them falling has groupwide consequences, so maybe consider making it a simple obstacle and have the tester be the lead mouse at the top.
On the other hand, if you were testing Nature to sneak past some soldiers instead, having each mouse test individually makes it more tense (and fun!). If one of them fails, maybe only he gets the attention of the guards, or maybe the alarm is raised and the whole patrol is discovered. It’s up to you.
Hope that helps.