ezyjet
I think Getoutofmy galley gave you a pretty comprehensive reply but you give me the impression you think there are some special rules for crew when they are home....
As airline crew ( at least in the UK) as Cabin or Flight Crew, you are not signing up to join HM forces or similar, i.e. organisations with quite rigid regulations that apply to where you can live and what you can do, even when off duty. Crewmembers are just members of the general public who happens to fly for a living, so off duty they can do whatever they want as long as they don't break the law or bring the company into disrepute. So to answer your questions, yes, you can share lifts (but in general Crew generally scatter to all points of the Compass at the speed of sound once they are off duty at base), yes they can stay together, move in together...it's really that simple.
If you as an individual decide to live a long way from base (and a lot do, especially Long Haulers) then it's up to you to make arrangements to be rested before duty (some do indeed bunk down at a friends, otherwise you'll have to find a cheap hotel or B&B near the airport).