Hey kyriakosdz,
If the groups are open/public then you can use the plugin to display them on your site. All you need is the ID of the group and you can then add it to the shortcode:
[custom-facebook-feed id=1234567890]
Make sure to set the Page Type setting on the plugin’s Settings page to be a ‘Group’, or you can set it in the shortcode itself:
[custom-facebook-feed pagetype=group id=1234567890]
In regards to any legal case, I’m not a lawyer and so don’t take my word as legal advice, but I’m not aware that it’s against Facebook’s Terms and Conditions to display any of their public posts. They make the data freely available via their Graph API. I would imagine in a worse case scenario the owner of a specific post may ask you to remove it from your site, in which case you could do so very easily.
If you have any other questions then just let me know.
John