[Alfa1]

You have to think about what the purpose of these functions are for your site. Simply adding a blog and groups may leave your members in confusion. My groups where left to be, asides from some members affirming they liked the same thing(music, food, whatever). That way there was not much difference between profiles and groups. Just meaningless chatter cause no one knew what else to do with it.

But when I started to use groups for common projects, that's where it became useful. Members started to check group posts regularly and it now is an important part of my board.

With blog, you will have to think about whether you want it to be a free for all where anything goes, or you want specific content to be posted there. Like journals, articles, monologs, etc. I think a free for all will get more activity, than specific content.
Make sure your blog is easily accessible from your home page.