Lighttpd

It's often share common user directories, and directories for a number of users, and a single common password file.

Could use a file. htaccess in each directory to protect, specifying a file in each directory .htaccess with the restrictions to that directory, to only allow users to access required and have been authenticated.

.htacces (for the directory /project_1)
AuthType Basic
AuthName "Restricted Access"
AuthUserFile /path/to/secure/file_htpasswords
Require user user_1
Require user user_2
Require user user_3

.htacces (for the directory /project_2)
AuthType Basic
AuthName "Restricted Access"
AuthUserFile /path/to/secure/file_htpasswords
Require user user_1
Require user user_3
Require user user_15

.htacces (for the directory /project_private)
AuthType Basic
AuthName "Restricted Access"
AuthUserFile /path/to/secure/file_htpasswords
Require user user_private

Please accept my suggestion.
Sebastián Segura