This may be the hardest part for some introverts. You've created your safe haven from the world and now you're going to invite the world into it? Start small, ask someone you really like over for tea or coffee. You can either give a verbal invitation or via email. If the two of you communicate via text that also works. Since I know you're not making a lot of phone calls, I would not suggest that you make a special call, that might make them nervous, same goes for snail mail. If those forms of communication are rare for you, avoid using them to invite your favorite friend over for a cuppa.
Alright! You've mastered one friend, now maybe a few friends? Let people know who else will be there when you invite them or at least who else is invited. People don't like surprises for coffee for the most part. And just because someone is your friend, it doesn't mean they're your friend's friend. And a frenemy in the midst is not conducive to being real with one another. Take your friends' relationships with one another into account when inviting small groups over. It's one of the more difficult things in life to do well.
You survived small groups, on to open houses? I don't think this is as common as it used to be. First of all, you have to have a group of people that like coming to your house OR have a common interest outside of you (workplace, club event, etc.). Flyers work well for open house invites, little postcards (these are especially useful because you can hand them out at will and tuck into a pocket without getting lost), or a verbal invite followed by written confirmation of date and time. We generally invite people a few weeks in advance and talk about it, remind people the entire time leading up to it. Part of that LRH system. Just make sure you don't get fixated on who says they'll be there, instead let those who decline know that you'll miss them.
Inviting folks isn't that difficult of a thing, once you get over the hump. We all used to invite people to spend time with us when we were little. As long as you don't make it a big deal, it won't be. The important thing to remember is to have fun, even in the inviting!