Hi, Gaile --
You're absolutely right: It is a delicate balance of too much info vs. enough.
The first page that a new member sees after verifying their email address looks like this and contains a number of links to additional information:
The link for "Family Member," takes them to the membership page: https://www.wikitree.com/wiki/Help:Membership#Family_Members
The information is all there for them, but they have to take the time to read it. So, if you happen to notice someone who is asking about different types of membership, and they're still listed as a guest, you can suggest that they go to My WikiTree > Nav Home Page.
Also, on their profile page, there is an "upgrade" link below their name. That same link is also next to the menu links in the upper right so that it "follows" them on any page they visit.
We've found in the Greeter Project that by keeping things simple by just connecting them with someone who can answer questions, we avoid overloading them with a bunch of text, and the Greeters are able to point them to what interests them most.