Let's assume that there is a guy user name paul who's main group is developer.
His home directory belongs to paul:developer.
Now, let's assume my main group is users, and the developer group is my secondary one.
You would expect, probably, that I could do cd ~paul if only paul's home directory had g+rx attribute, right?
Well, on HP-UX (at least) it does not have to be like that. I don't know if this is by default or not but what I have found out on our HP-UX machine is that there are two files in the /etc dirtectory - group, and logingroups.
For sure you know the /etc/group file!
I bet you might not see the /etc/logingroups file, though.
Now, if you want to make HP-UX behaving like other machines, you can simply create a link:
ln -s /etc/logingroups /etc/group
After this change my user will be able to do cd ~paul.