Symptoms

How to set permission on Custom Workspaces ?

Diagnosis

Currently it is not possible to restrict access to the Custom Workspaces for the specific users.

Solution

If you are using a form link when configuring workspace,  you can restrict the access to the form using K2 Management.

Go to K2 Management> Categories > select the Category > select Form
You can Add /Remove Form Authorization to the users from the Security section, Please follow the documentation on Form Authorization for more information:
https://help.k2.com/onlinehelp/k2five/userguide/5.0/default.htm#K2_Management_Site/Catagories/Forms.htm#Form_Authorization

When the users who are restricted to access the form, click on the Custom Workspace link, an error message will be displayed:
"Form 'FORM NAME' could not be found. Ensure that the Form exists, that it is checked in, and that you are authorized to run the Form."

Please follow user guide to learn more about Custom Workspaces:
http://help.k2.com/onlinehelp/k2five/userguide/5.0/default.htm#k2_management_site/user_management/workspaces.htm