Using a public GIT-Hub repository (that was a homework of my group) in my Bachelor thesis
Do I have to mention my colleagues, who implemented that homework with me, explicitly in my Bachelor Thesis?


Depends on what referencing system your given institute uses. Most likely, it would be a good idea to mention that in your reference. For example, if you are using the Harvard system:

[Surname], [First-name] et al [System (e.g. Github)] [Year of creation], [Repository Name], [Version #], [Type of 'thing'], accessed [Access date], [Repository URL]



  Do I need my colleagues agreement that I can use this project for my Bachelor Thesis?


As long as you correctly reference it, and that you don't use it commercially you should be fine, but just doing something like emailing them asking if you can use it as an example for your assignment would do no harm.


  Does it make any legal difference if I provide a link directly to this sample project or if the project is just visible in screenshots without showing an explicit URL?


Legal difference? Yes. If it would ever come to such, giving the actual URL would/could effectively protect you, as the opposition could claim that you edited the screenshots or that they weren't of the correct repository in the first place. 



To be honest, I would prefer to have a few extra words in my bibliography rather than be taken to court and be prosecuted.