Team Members: There is a limit of 100 members per blog.A member can be either an administrator, an author, or (if the blog is private) a designated reader.
In the help forums we have a dialogue where Blogger Employee admitted to a recently imposed limit per blog, of 100 Administrators / Authors / Readers, to reduce server overload.
there were lots of performance issues that arised as a blog amassed more than 100 readers.
If you have a private blog, you'll be able to have a maximum of 99 readers, since every blog has to have at least 1 administrator. With a private blog, the membership invitations include a temporary membership token that lets prospective members view the blog, without joining.
It appears that the use of the token may not be consistently subject to the 100 member limit - but be aware that the temporary membership token has an expiry, built in. If you base your membership access on use of the token, don't be surprised when some members occasionally complain of being locked out.
And even as severe as the limitation of 100 seems to some, other bloggers would be happy to be able to exercise that number.