Teams are a wonderful thing — when everyone is linked together and plays off of each others strengths an IT department can virtually hum just like a smooth flowing machine. However, when a team member decides to leave the team, whether its to go to another internal team or to go to a new company, it can cause the whole department engine to start to sputter and misfire.
For those who are left on the team, the loss of a team member who has chosen to leave the team, can be difficult to deal with. A good IT manager has to recognize that envy will immediately show up: we all are jealous of others who appear to be heading for green pastures/better challenges/more money. The most effective way to deal with this is to let it happen, wait for the person to actually physically leave, and then reveal / discuss new exciting challenges for the remaining team. The envy is a short term emotion and can be quickly buried if you can get the team excited about another project task.
The next big question is how does the loss of the team member impact the team. Specifically, who will be picking up the work that the leaving team member was working on? Additionally, does this loss of a team member open the door for any promotions? Answers to both of these issues need to be determined as quickly as possible and communicated to the whole team early on.
There is a lot more to do, but these are two good places to start.