Not only do you need membership to post in a given blog, you need membership to read some blogs. This is an excellent improvement in blogs, where the prospective readers know how to contact the owner.
What if you wish to read a given blog, and you don't know how to contact the owner? You're out of luck, if you don't feel like doing some detective work to identify a contact point.
Maybe not, if the blog owner has some foresight.
If you are the blog owner, and you want to have a private blog, that's made available in a limited fashion, make it easy for your readers.
It's simple to setup a second, public blog, with an invitation.
Blogs are free, so setup 2 blogs. Give the public the address of one blog, and leave it public. Put a single welcome message in the blog.
This is my blog, and entrance is by invitation. Send an email to email@example.com, requesting entry. If I feel like granting you entry, I'll send an invitation back to your email address. Make sure that your email service can accept the invitation, open the email and accept the invitation, and you're in.
Or, setup a post, and let people comment.
An alternate solution would be for you to setup a post in the public blog, and have people comment on that post, which you moderate.
This is my blog, and entrance is by invitation. Leave a comment with your email (will never be published), on my moderated post Leave Comments Here, requesting entry. If I feel like granting you entry, I'll send an invitation back to your email address. Make sure that your email service can accept the invitation, open the email and accept the invitation, and you're in.
Then. combine the two blogs.
You send invitations for membership in the second blog, which is private. Your prospective readers can join, using whatever account they wish, only after you invite them. You can use many different techniques to make the two different blogs look like one blog.
Your readers can bookmark the second blog after they get in, and everybody is happy. Except, of course, the unlucky ones who can't get an invitation. But who said life was fair?