What do you mean by onboarding in that sense? Like, have an automatic "welcome" PM sent to them telling them the basics about the forum they've joined?
Well for me, I just let them figure things out for themselves and if they're stuck or curious, they can make a thread or directly contact me about anything, I don't mind.
I'll post in introduction topics, but that's usually about it. If there's certain topics they're interested in, I may point them in those directions or ping members who are knowledgeable on those topics to come say hi too, but even that was always relatively infrequent.
I send new members a welcome PM and let them know that we’re glad to have them on board, and that they can contact me if they need help with anything.
I also point out the best areas to post in the forum, including where to introduce themselves and which sections are designed for specific discussions.