A "member" in Church Directory, is any man, woman or child, at your congregation, that belong in the church directory.
This is not to be confused with being a "Christian", which is identified by your congregation.
When someone is a member of your congregation in Church Directory, they have access to login to Church Directory and access your congregations' account; as long as they have an email address on file for their user account. (Or have setup alternate login options)
This flag on a Role can be set under the "General Settings" tab when editing a Role, and selecting the appropriate checkbox. Here's an example:
NOTE: This method is only a Role that determines membership access.
It is highly recommended to follow some best practices for this special flag on Roles.