Although there are three different levels of access set by default, it is possible to customise access to include a combination of these roles.
Ensure that you are in the relevant blog
Click on the Settings button and select Permissions:

Click on the Create a Custom Role link:

This will take you to the following screen:

Check the relevant permissions from any or all of the default roles
The people who are already able to access your blog will be in the People and Groups box.
To add others: Go to the Add Custom Role section

Select from everyone, people, courses, organisations, My Courses or My Organisations by clicking on the relevant titles and using the next column to filter.
When you have made a selection it will appear in the People and Groups section

To invite someone who is not on the list, click on the Invite a person via email button

There is also the option to give people restricted access for a limited time.  To do this, enter a start/end date/time as appropriate.

When you have added the relevant people, click on Save Click on Save &amp  Exit when you are done