You can grant access to everyone you want to all of your appartment just by adding there names on the list on http://www.neocron-game.com/, just log in (your game's log/pwd) and the menu comes in ('My account').
They can comes in but can't use cabinets.

For the clan appartment, you need to be a member of the faction, put you LE off (if you haven't already did that), and just buy the clan's appartment key to create a clan in that faction, get this appartment for whatever you need to do and invite whoever you want into.
But poeple that are not from your clan can not use cabinets, using a clan appartment in this case is not different that using a normal appartment (and normal appartment are 'better' ^^).