@LKass , @JSyq , when scheduling your meeting, select "Show advanced options > Scheduling Options." There you should see the option "Join before host". If you don't see this option, you'll have to contact your administrator in order to activate it.
If the option has been activated by your administrator, it will be enabled by default and set to "5 minutes before start time". As a user can disable it and/or change the timer to 0/5/10/15 minutes.