Hello
How to assign joomla groups to k2 "user groups" ?
Some times there is too many registered users in website that are assigned to joomla user groups and registration in continued . assigning users in k2 "User groups" one by one is informal . if we able to assign joomla groups to k2 user groups it will be very useful for this case.

Hello,
Check the K2 User plugins' status.
If it is enabled all new user are assigned to the default K2 User Group.

I know
But I mean assigning joomla users to k2 user groups via assigning joomla user groups to k2 "user groups".

Unfortunately this is not possible.
K2 and Joomla! User groups are two separate things.