Inviting happens after I've gotten to know a bit about the person, their background, hobbies, current life situation, a few photos, etc. Usually this was done through back and forth emails. "What's up" is a start of a conversation. If it stops there, that means people are not that interested.
Not to say there aren't passive guys, but they would offer up possibilities if they want to be invited to something. i.e. "I'm into live music", or "I like long walks". If you are really interested in them you should invite them to do one of these things.
The people the are not invitable are those who are passive, but they tell you up front what they DON'T like, and they have a long list of it. Indicates a passive-aggressive personality. Or those who are narcissistic that they're not reciprocating attention and respect to others.