Releasing project team members is not an official process. However, it should be noted that at the conclusion of the project, you will release your project team members, and they will go back to their functional managers or get assigned to a new project if you're working in a matrix type organization.
You will want to keep the functional managers or other project managers informed as you get closer to project completion so that they have time to adequately plan for the return of their employees. Start letting them know a few months ahead of time what the schedule looks like and how soon they can plan on using their employees on new projects. This gives the other managers the ability to start planning activities and scheduling activity dates.
Celebrate!
I think it's a good idea to hold a celebration at the conclusion of a successfully completed project. The project team should celebrate their accomplishment, and you should officially recognize their efforts and thank them for their participation. Any number of ideas come to mind here—a party, a trip to a ball game, pizza and sodas at lunch time, and so on. This shouldn't be the only time you've recognized your team, as discussed during the Team Development process, but now is the time to officially close the project and thank your team members.
A celebration helps team members formally recognize the project end and brings closure to the work they've done. It also encourages them to remember what they've learned and to start thinking about how their experiences will benefit them and the organization during the next project.