Move to vRO.
Looking at the plug-in code, it seems that service groups are not part of the plug-in object model. If this is indeed the case, then you won't be able to create a service group, t least with the current plug-in version.
Of course, it would be good if NSX plug-in developers confirm this, but I'm not sure if they are monitoring this forum, so I'd suggest to repost your question also in NSX forum at https://communities.vmware.com/community/vmtn/nsx
Thank you for your reply.
I will ask there as well.
In case they confirm I have to develop a plugin I guess
You can probably use the rest plugin to fill in the gaps in the vRO plugin.