You could try and put all sorts of smells cats dislike, from peppermint to
pet-shop sprays to lemon, but it wont stop them. No way.

I lost my QT outside my house 5yrs ago 3 weeks before his 1st b-day. He got
run over in front of my eyes. I'm still not over him, never will be.  I
stopped letting my others out freely back then.

The only way I let mine 5 out now is  on a harness and leash. You can buy
lovely cat harnesses in most pet stores and a retractable long dog leash and
let your cat take you for a walk.

Its not a perfect solution.But its better for everyone, rather then her
dying.

The only other solution is to fence up the garden, but it would have to be
high up. let me see if I can get some links for ya:
http://www.cat-world.com.au/cat-worldenclosures.htm
http://www.catfencein.com/
http://www.cat-world.com/cat-worldenclosures.htm
http://www.scvhumane.org/behavior/cat/cat_fence.htm
http://www.just4cats.com/index.html
http://www.lisaviolet.com/cathouse/backyard.html

(Hopefully they still work)

Finally the last option is to keep your cat indoors. It may complain at
first but they get used to it. Just make sure you provide some surfaces for
climbing, some scratching posts and some company in a form of a cat
preferably.

As for cat grouping, yes cats like to mingle as well and will often make
friends with neighbours cats. Or they could also fight and catch some
disease.

Gee