Here's how to "kill" an active session of one concurrent user so that someone else (within your license limit) can start
The Engage Process subscription of your organization determines how many users can use the Engage Process Modeler tool in the same time. For example, if you have only two concurrent licenses for Engage Process Modeler and currently two users are using the tool then, at the moment the third users wants to login, his attempt will be rejected with the message 'Too many concurrent users'.
Actually, what is checked when a user wants to login is the number of active sessions. It may happen that if you did not close your browser session correctly your own old session is still active. This situation is recognized by the tool and you get the following page.
Click the 'Kill session' link to end your own Engage Process Modeler active session and to create a new one within the license limit. Or, if the Engage Process Modeler is still opened by you in another browser, close it there and click the 'Retry' link.
It may happen, however, that not you but one of the other users did not close his/her session correctly. This session is still active and it blocks one of the concurrent licenses of your organization. You can ask the other user to login to Engage Process Modeler again, kill his old session and close the current one correctly. This, however is not always possible.
If you are in the Users Manager role you are allowed to kill sessions of other users. First, open the Engage Process Admin Center tool by pressing the application launcher button 
on the top-right of the window and then select Admin Center (you can login directly to it using the link https://admincenter.engageprocess.com . Fortunately, you do not need a concurrent license to open this tool. Select the Active session tab there.
In the table that is shown you see all the active session for your organization including Engage Process Modeler, Engage Process Viewer and Engage Process Publisher sessions. Select the session that you want to stop and press the Kill session button on the right hand side.
Press the Refresh button on top if you miss some sessions in the list.