Lots of good points. Just to add to #1 about stuff to do, a lot of times introverts will not just invite themselves into an activity. You can tell if they do want to join by asking, and usually if they do, they'll jump on opportunity. If they don't, you'll notice hesitation. Sometimes even hesitation to say no to avoid awkward situation. It's always nice to give them an out if you notice hesitation, like "if you want to". This will also help gain trust and will show that saying no will not be awkward. Less uncomfortable moments = more likeliness of saying yes to stuff.