ISSUE SUMMARY: When a user wants to register via Jomsocial, a message is displayed that the user has to login first. In other words, it's impossible to register for a new user. When I use the native Joomla registration form, everything works fine.

However, when I disable the SEF option in the general control panel of Joomla, users can register via Jomsocial.

How can I use SEF and let new users register themselves via Jomsocial?

STEPS TO REPLICATE:
1 goto youngold.nl
2 click on "Registreer" in the upper right corner
3 enjoy the message (in Dutch) "login first"
4
5
RESULT failure to register
EXPECTED RESULT An empty registration form
BROWSER Chrome

HI, Cor.

Thank you for contacting us and reporting this issue.
I fixed it on your site ;)
Issue was caused by menu misconfiguration: you didn't have JomSocial frontpage menu item on your site this Joomla! took as default first menu item that was created as community page.... and that was a profile page. Profile page could be displayed only for registered users so when you tried to register... you ware asked to login. Funny, isn't it? :)
So I created a new menu item in "Stealth menu" "Community" that display community frontpage and is public. Now when you try to register - registration form is displayed properly ;) And this works perfectly with SEF too.