To invoke an item while adventuring, select the character you wish to have invoke an item from the drop down list in the upper left panel and then hit the 'Invoke Item' button. If the character is carrying any items that may be invoked outside of combat, you will be asked to select which item to invoke, and, possibly, whom to invoke it on. If an item may only be invoked one time, it breaks after the invocation and is removed from that character's inventory.

Items designed for combat use can only be invoked when in combat. See the Combat help page for more information.