some advice I got given years and years ago when I was still in high school was, you do not need to invite them inside, but offer them a drink and a conversation, (try to keep it away from the bible... when I have done this I have been sucsessful some of the time in doing this) and the mentor who was giving this advice told me he always offered them his phone, because often they would not be allowed to call home while on mission, and he wanted them to be allowed to talk to their family.

This advice has really stuck with me, and I have tried to do this every time I have had interactions with them before. My best experence me and one missionary where talking about a shared intrest over aircraft, it was a good 45 minutes, though it was unfortunately ruined when his partner noticed that we where not talking about religion.