I am currently using joomla 2.5 and k2 version 2.5.4 although it happened in Joomla 1.7 and previous versions of K2.... in the past, I just dumped k2 and went other routes but I like K2 (except for this issue) and hoping there is a solution. It has to do with Comment form on a K2 Article.

If I have the K2 – Articles – Parameters – Comments set to Enabled for everyone… the articles allow comments… comment form is there - I see the comments - no issues… but if I have the Global Setting for Comments set to Enabled for Registered Users Only – I do see the comments previously made but I cant comment when logged in i.e. there is no comment form.

I log in with a Registered user… I have the K2 Group Settings/permissions for the Registered group to be able to Post Comments. I have a “standard” setup / install using Rochen as my server… pretty much standard stuff. There is only 1 group for Registered Users…

This has occurred on other installs and other templates… I see others have the same issue. The user is in the User manager… for K2 and the Joomla User list. I have tried logging in using K2 login and Joomla’s login. There is no logical reason, other than a bug that I can see being the cause for this... but how it usually is - probably a permission or simple setting.

I am having the SAME EXACT issue! I just published our news section and thought it was all working because I was logged in as an author and could comment and see the box. All the authors are able to see it and comment but regular registered users cannot.

Edit:
I am on Jommla 1.5.25
K2 2.5.2

Ok I had to manually add all 568 users to the Registered User group. I am using JomSocial as my registration component and it is not placing the users in the group automatically.. You need to have your users sign up with the K2 registration form for it to automatically add them to the Registered user group. Also go to User Groups and select Registered and make sure that post comments is allowed.

Should be an option in K2 to sync with JomSocial profiles so that it automatically displayed the user as registered.