If you want just one particular user to generate an alarm when they open a site, and a restoral when they close the site, you can override the action plan at user level. This is done from the Users tab of a client, under the Client Details heading.
Insert 2 action plan overrides:
Other users will be able to open and close the site normally, but for this user the opening will generate an alarm. If you do not want this user's opening and closing events to change the site status, you need to use an action plan with signal type of User Info, so the name of the user appears in the signal description:
This will mean that the user name is shown in the signal and the alarm descriptions: