This can happen if you delete the default post-DEpendent member group "YaBB Newbies".  The issue is, when a new member joins they have a post count = -1, and unless there is a post-dependent group set up to accept them into, the new user will be a member of NO group at all, and therefore will not have access to anything beyond what a guest can access.  It means they won't be able to post either - unless you have guest posting turned on, which isn't a good idea.

To fix this, you must have at least 1 post dependent group and the minimum post count must be set at -1.  

For help setting up a post-dependent member group, see the Video Tutorial available here.