If someone managed to wander upon our place, and I didn't know them, they would be turned away. I'm not doing this to become a charity. Everyone has access to the same information about world affairs' as we do, so its their problem if they chose to ignore the signs and do nothing.
For me, this all depends. I like to think I'm fairly decent at sizing up people. Thing is, assuming all of my friends and family who would come (vs. staying in their own place), could come, that'd still put us at about 10 people. Post SHTF, you'll need people, to both defend what you have, and to provide enough for all, as well as to have skill diversification. At least half of us 10 aren't the youngest or most fit (though I assume we'd be more fit after such an event), so new blood could be a good thing.
Having more people to patrol, loot, grow food, etc. is a good thing. As long as the group can provide for them, and they are willing to do their part. I can imagine, that for a lone traveler who stumbles upon us, this would probably be a godsend. I know if I was foraging through the woods, then was able to eat a hot meal, take a shower, and sleep on a bed, I'd certainly do all I could to become part of such a group.
That all said, you have to consider the possibility that they are NOT alone, and just scouting out for a group...so you take precautions. I have some pretty decent plans for this, though I'm always refining, and of course, no situation is the same.