How to create different levels of registered users

Hi I have
Social, Blog and Discuss.

I want to create 2 types of users:
- 1 those who can post in Discuss
- 2 those who can use Social and Post in Blog

I dont want 1 to have access to Social Dashboard and Blog posting. But I want them to be able to read blogs.

Questions:
- how do I do it?
- through Social Dashboard can I have both Blog and Discuss being accessible?

Thanks

Vlad