I'm using Joomla 2.5.1 with K2 2.5.5 and I'm trying to set up some users.

By default the "user groups" tab is empty, so I create a new group "editor".
Now I add one of the users (that I imported from Joomla users) to this new "editor" group and give it all permissions (add items, edit items, etc).

But when I log in as this new user, I can't access any of the k2 functions ("items", "categories", whatever).
Instead, I get the following error message:

"You are not authorised to view this resource."

Any ideas what could be wrong ?

Be sure to disable the default joomla login module and enable the k2 login and or k2 user module to log in
after logging in check the login module and see if there is an add item link
also, be sure that you assigned categories to the user group

Maybe I forgot to mention one thing:
What I want is to allow one or more users (apart from the default administrator) to login on the *backend* (i.e. /administrator) and perform some trivial tasks, such as addding/editing items without having access to all the other administration stuff.

What I don"t want is to put a login module on my frontend.
But what you suggested is related to the frontend login if I'm not mistaken ?