Mentorship, having fun things to get involved with and relaying information about the right communities with specific interests for newcomers to start with will make it feel like they are part of something.
I've managed to help a couple of people who posted for the first time and read their intro post to see what their interests are and point them in the right direction in terms of communities but obviously the comments required are customised depending on what that person is interested in.
I found that made a big difference and being around to comment on their posts for the first couple of weeks as they "bed in", leaving some information in the comments was a good approach.
Probably knowing where to look and ask for help is the biggest thing, especially when you stumble in to this place for the first time, otherwise, it's off putting and a negative first impression sticks...