Elgg has partial support for so called "multisite". I guess the best way to get an understanding of it's status is to read these issues: https://github.com/Elgg/Elgg/search?q=multisite&state=open&type=Issues&utf8=%E2%9C%93

Thank you for your answers.

Please @Matt can you send me your Vagrantfile and if it's possible to tell me what's the path of the dataroot that you use (I choose always the folder in the folder shared with the host machine and the vagrant box - I didn't use NFS to share the folder , I use the basic usage https://docs.vagrantup.com/v2/synced-folders/basic_usage.html ), I didn't modify the .htacess delivred with Elgg installation.

Thank you so much for you help.

I've documented everything on my site, mostly for my own reference for when I need to rebuild stuff but if it can help others so much the better.

http://known.mattbeckett.me/content/all/?q=%23vagrant

I use vagrant mostly because we've run into situations where different projects require different environments so I can maintain that in separate boxes whereas XAMPP I couldn't.  There's definitely a performance hit with the virtualization though, unless you're doing multiple projects with different incompatible server settings then it's probably easier to do it in XAMPP

Thank you so much Matt, I will give it a try.

This can't be done, sorry to say.

At some point recently we started ignoring the river.access_id, and only show items where the user can see the object and target entities. That was probably not properly marked as a breaking change.

Well, you can use the "wheres" option in elgg_get_river() to check for the access_id like the older versions did.

Thank you very much for the prompt reply. 

Account-specific friend collections can be created from:

The friends icon in topbar > Friend collections > New collection

If the ACL needs to be site-wide (not used just by a single user, but all three users) I recommend instead creating a closed group and adding those three users as it's members.

Thank you Juho,

I am programming this feature ,so I can't use UI to add friend or group.

What exactly is your goal? What is the car objects used for? Who are the three people and why only they are allowed to access the content?

Is it an idea or a guidance allow me to customize sidebar like hide file menu or blog menu for example ?