We add almost all new members to a group called “Newbie Peppers”. They stay in this group for at least two weeks as they learn their way around the community. Some categories are not open to this group to see. Once they have been in the community two weeks, they can then request entrance to the second group “Seasoned Peppers”, by answering the questions and requesting to join. This is the group that sees almost everything (except a few things reserved for Admins, Mods & Level 4’s).
The topic we want all new members to see first is one we have called “Some information for Newbies”. This details the things they should do first when joining our community and provides direct links to specific other posts we want them to read, including rules, tips & tricks for navigating our platform, etc, etc. etc. It is not enough to pin the topic globally, as 1. we have found they do not find it and 2. Once they are familiar with the community they no longer need the info in that post and pinning doesn’t seem to work well for us. Discourse is very new to most of them, they feel lost and we try to ease the frustration of learning a new platform (and the same time that they are very possibly brand new to our topic too) by providing some guidance step by step so they don’t feel so overwhelmed. [Sometimes that works, other times it doesn’t - lol - but we try].
New members jumping into the list of popular topics doesn’t provide the step by step things to do and they then miss those steps.
Additionally, due to the complex nature of our topic, sometimes they will sign up, do a couple things, and then come back later to do more, completing the steps from that post over time rather than right away.
I hope this helps explain why we need the ability to control the first post and the specific group for new members. Let me know if I can explain anything more 
.