Add Board you are in to a Board Collection
When you are in a Board and you would like to add it to one or many Board Collections without leaving the Board you can open the left panel.
Then choose the Board Collection you would like to add this board to and click on the 3 dots after the name of the Board Collection and choose the action: Add current Board
In the following screenshot we are adding the "Vacancy Management" to the Board Collection "Marketing"
Once added you will find the Board in your selected Board Collection inside of the first Group on first position. From there you can move it to another Board Group or change the order of boards. More about this here: