After an item is created you can always change its workgroups assignment.
Go to the item screen for the subject meeting.
On the left side of the screen, find the Item Data block.
Find Item belongs to the workgroups. If your access privileges allow you to change the workgroup assignments, the Manage Item's Workgroups link will appear. Click it.
In the Select Workgroups box, make the workgroup assignment changes you want by selecting or deselecting the listed workgroups.
Click the Set Workgroups button.