Depending on your subscription, you have a maximum number of sessions available at a given time; only that many users can be logged in at the same time. For example, the 1-User subscription allows only one person to be logged, whereas the 5-User subscription allows up to 5 users to be logged in at a time.
You can manage these sessions by clicking “Active Sessions” from the “Account” drop down. The table on this page will show the details for the sessions currently active on the account. From here you can either select to end a single session or end all sessions (except your own) with the click of a button. Please note that doing this will automatically log those users out of the account, even if they are currently using the search.
If you are attempting to log in when the maximum sessions has already been reached, you will be prompted to clear out a session before you can log in.