Q:

Create a group for branches

How can I create a group of branches with command line, Sourcetree or Gitlab?  With group of branches a folder that contain all the branches from the same group, "feature" for example:

A:

In git, what you're asking for is called a "namespace".   The way I usually see it used, it's mostly a glorified naming convention; but it can be a bit more than that, as outlined here: https://git-scm.com/docs/gitnamespaces
You can name your branches like feature/newThing, feature/coolFeature, feature/bugFix, etc.   Internally this does organize them into different folders on the filesystem (for what that's worth).   More importantly, you can then address all branches from a namespace in a refspec rule, among other things. 
If you're coming from a system like TFVC, where branches are literally organized into folders, this may seem a little flimsy; but then most branch concepts are lighter-weight in git than what other systems' users are accustomed to.