userExists
Description
Verifies if the selected user is registered Jira user.
Parameters
Return Type
Boolean
A "true" return value means that there is a registered Jira user associated with the specified username or userkey.
Example
userExists("Administrator");Returns "true" if Administrator is the username of a registered Jira user and "false" otherwise.
The look-up is first made after the userkey, then after the username.