The configuration of the apache server that is running from MAMP is done below
/Applications/MAMP/conf/apache/
The first thing that needs to be done is to load the configuration files of the individual users. This can be done by uncommenting the line where the file httpd-userdir.conf is loaded. Hence, the first step is to uncomment the following line:
$ cat httpd.conf | grep httpd-userdir.conf
Include /Applications/MAMP/conf/apache/extra/httpd-userdir.conf
$
Now, under the directory extra one can modify the httpd-userdir.conf file so that it is loaded with the options that you want. The first observation is that the httpd-userdir.conf that comes with MAMP has a line that reads:
UserDir public_html
Notice that this is not the expected directory under OS X. Hence, I would suggest to change that line and make it read like this:
UserDir Sites
since "Sites" is the expected directory for this purpose under OS X. Otherwise you should notify the users of the system that you are using public_html just like in traditional unix/linux distributions.
Similarly, further below in the httpd-userdir.conf file you should change the line
<Directory "/home/*/public_html">
to
<Directory "/Users/*/Sites">
Note that the MAMP version of httpd-userdir.conf is different in that respect compared to the default httpd-userdir.conf file that comes with the apache that is installed with OS X and can be found under /etc/apache2/extra/ . Compare these two files and use which ever version you prefer for the configuration. Personally, I would go for the default configuration that comes with OS X, meaning that I would use something like this:
#
# Users might not be in /Users/*/Sites, so use user-specific config files.
#
Include /private/etc/apache2/users/*.conf
<IfModule bonjour_module>
RegisterUserSite customized-users
</IfModule>
since it gives you even more flexibility for the configuration of the individual users.