No matter the template I use if in K2 settings I set the option for comments in ENABLED FOR REGISTERED USERS ONLY there's no way to visualize the module to insert the comment, nore for SuperUser or anybody else.
In this case a message appear saying " YOU MUST LOGIN FIRST TO WRITE COMMENTS" at bottom of k2 article.

Instead if I leave it to ENABLED TO EVERYONE the module appears with no problem.
Whould You please let me know why ?

Many thanks Fotis,
In fact what I need is only a group of registered users can post and read comments.
I do have reCaptcha enabled on the form, but reCaptcha does not resolve the matter, to me it's just an anti-spam and does not avoid anyone, even guest, to post comments.
Still I do not understand how to fix it:
- ENABLED FOR REGISTERED USERS ONLY : no comments form appears and nobody, even super user can post
- ENABLED TO EVERYONE : the form appears on any, and not on only items/categories I would like, k2 article and everyone including guests can post comments

Would you please be more clear on how to resolve the matter ?
Many thanks in advance

Many thanks
But Yes the group "friends" is part ok K2 groups and has enabled comments.
Yes on my site there are two extra components:
- Simple Membership component
and
- User Login One plugin

In case this does not help, please contact me directly and I'll provide you an admin userid + passw to look in the site.

Many thanks in advance