How can I remove/delete a user from a K2 group without them being deleted from Joomla ?
Cheers

You may have to use phpmyadmin and change the value in the group field to blank in the K2_users table, that will leave the joomla user

That's a pain, We've got 2500 users in an K2 group that we don't want in that group. For some reason only these 2500 have been added to K2 when our other 30,000 haven't been.
How are you supposed the manage users in K2 if you can't remove them from a group without deleting them ?
It's pretty poor show that I can't remove users from K2 with editing the database , it's also not made clear that deleting a K2 user deletes them from Joomla (and in our case the phpBB forum as well ! )
What happens to these users in K2 if we uninstall K2 ?

K2 users are normal Joomla! users with some extra data.
If the K2 User was not enabled, Joomla! users were not imported to K2.

Finally you can batch move users by selecting them and clicking on the move icon.

The K2 users plugin may have been enabled at one stage but it isn't now, which may possibly explain the 2500 users that are in K2 as we certainly didn't deliberately add them.
Batch move only allows you to move users from one K2 group to another not take them out of groups, unless I'm doing something wrong or missing something.
We have two K2 groups, Authors (2500 users) and Editors (4 users) we need to take all the users out of Authors and 2 out of Editors, this seems to be impossible without modifying the database which I think is a bit much.
What happens to users in the 2 groups if we delete the groups ? or were to un-install K2 ?
Cheers

If the plugin is not active, then the user is not added to K2.
If you uninstall K2 you will lose all your content, there is no going back from this.

Create a new group with no permissions, set it as K2's default user group (check K2's params) and batch move users to that group. Remember to move 50 - 100 users only at a time.

Krikor wrote: If the plugin is not active, then the user is not added to K2.

I gathered that from Williams post and have disabled it

Krikor wrote: If you uninstall K2 you will lose all your content, there is no going back from this.

I take you mean K2 content, not Joomla or phpBB content ? although seeing as deleting K2 users deletes Joomla users I'd like to be sure!

Krikor wrote: Create a new group with no permissions, set it as K2's default user group (check K2's params) and batch move users to that group. Remember to move 50 - 100 users only at a time.

By experimenting I've already arrived at a solution similar to this.
I found that if you delete a K2 group it doesn't delete the users in that group from Joomla, they are left n their default Joomla groups. So I created a temporary group (no permissions same as you suggest), batch transferred 50 users to it, then deleted the temporary group, repeated as necessary, took a while but I got where I wanted to be.
Small suggestion, I think it would be worthwhile making it VERY clear in the K2 documentation that deleting a user from K2 deletes them from Joomla., could very well save someone some heartache, lack of this information certainly caused us some user problem which luckily we were able to rectify. The Part of the problem it caused us is that we have a phpBB3 forum in a Joomla wrapper with a bridge module synchronising the two which meant the users were deleted from the forum as well !

or were to un-install K2

This will remove all of your K2 items. As mentioned if the plugin is enabled, then users are being synchronised, although it is much safer to move the users and disable the user plugin if you don't need K2 users.