OK. How many times in multiplayer have you converted units and lost your spies as they are seen after the conversion, or clicked on a bunch of units but miss the last one and click the ground cancelling all previous orders.

Get them all together (control group or ; then home is your choice) I prefer a control group as i may have a spy elsewhere watching some territory and I want it to stay there. I use 7 as my spy control group (007 - spies) I never forget this. Anyhow you have the spies you wish to multiple bribe with. Hit shift then the multiple targets keeping shift pressed down if you hit the ground they will still follow their previous orders before hitting the waypoint, after the bribe que is set then click back to your army. Each individual spy will complete their bribe and leave immediately to the next way point saving many of them from dying. If you do not use shift and a waypoint you will need to reselect your spies as each bribe is made as you lose control of them when the bribe is completed.