There are a few different ways to group tracks and faders in Logic, this is a quick summary of what I've found so far.

So before I begin here I should point out that I'm a noob with Logic, so there could be some omissions (and maybe some mistakes!), but I thought it would be worth summarizing the options for those who didn't RTFM.

Folder stacks:

These are the simplest and most straightforward option: in the arranger view tap the multi-select icon and select a bunch of tracks, then de-activate multi-select, tap one of the tracks and in the menu that pops up select Create Folder Stack.

Your tracks are now in a group, and you can control the relative volume of the entire group with the fader on the Folder Stack channel.

You can't do any bus processing on this group though, the only control you have is volume.

The main purpose of this feature is to organise your mix: put a bunch of related channels into a group and adjust their levels together, and also collapse them so that they take up less space in the Mixer and Arranger views.

You can also nest Folder Stacks if you want to, however in my testing the volume of a nested stack doesn't do anything, which makes this less useful (unless there is an option to change this behaviour that I have yet to find of course).

Summing Stacks:

Summing Stacks are similar to Folder Stacks, with two major differences: you can't nest Summing Stacks (but you can have Folder Stacks inside them), and you can perform bus processing on a Summing Stack.

So if you have a set of drum channels that you want to organise and bus compress, put them in a Summing Stack and then add a compressor onto the stack channel.

When you create a Summing Stack, Logic will do some routing behind the scenes and create a new bus, and then point the output of all the included channels to that new bus.

Fader Groups:

This simply connects separate faders together so that when you move one fader, all the other faders in the group will move as well.

So for example if you have double-tracked guitars panned left and right, put them in a fader group to control both volume levels simultaneously.

To use Fader Groups you will need to make them visible in the Mixer view, by tapping on the options menu at the top-right of the Mixer panel.

Then you can tap the Group button and assign a group number to a fader. All faders in the same group will move together.

One benefit of having this additional option is that it helps to mitigate the issues with nested Folder Stacks. Grouped faders will give you an additional level of control over a subset of faders, so rather than nesting you can use Fader Groups within a stack for more control of your levels.

Routing in and out of a Summing Stack:

One apparent limitation you might encounter is the inability to nest Summing Stacks, but it is possible to work around this thanks to the flexibility of Logic's routing system.

Let's say you you have six channels of drums inside a Summing Stack. You've applied compression to the whole stack, but now you find that you also want to add some saturation to just the kick and snare, not the cymbals. Since you can't nest stacks you might think it's not possible to do this, but there is a way.

What you can do is to create a new bus, and route your kick and snare out to that new bus (rather than to the one assigned to the Summing Stack). On that bus you can then apply any additional processing you want, and then simply route the output of that bus back to the Summing Stack, by selecting its bus as the output of your new channel.

With this routing you can still control the volume of the entire stack with one fader, and apply nested processing on different elements within the stack.