I am using my Github page account as a blog website. Can I upload a powerpoint presentation on Github pages so that users can view it there? If yes, how to do that? I am a noob in Github. I also wanted to know if a group can collaborate on Github Pages? Thanks in advance.
I'm assuming you are looking for ways to incorporate your presentations into your blog so people can "page through them".
If you want to stick to PowerPoint or display old slides (done in PP), this existing Stack Overflow question might help: Embed a Powerpoint in a Web Page
You could, however, also switch to an HTML-based solution for future slides. reveal.js is a popular example, and it's even open source and all its source code is on GitHub. You'll have to check how you can incorporate HTML slides done with such a tool into your existing blog.
There are also slide hosting services for sharing presentations where you can upload your slides (in the form of PDFs, possibly also other files) and have them converted to an embeddable online presentation. One such service is Speaker Deck, which was incidentally created by GitHub (but acquired by another company only last month). Another popular service is LinkedIn's SlideShare. For these, you'd only have to add embed codes to your blog posts.
Finally, if you are fine with making your slides available as PDFs, which visitors to your blog would download (or view directly in their browsers, though this depends on their browser settings), you could do just as suggested in the other replies.
Your other question should really be a separate SO question as it's about a different topic.