Sharing activities is possible, but not recommended!
The activities are related to a stream. So if developers are working on the same stream as in this scenario It's technically possible. If the users where on different streams, they would not be able to share activities.
But even though ClearCase does not strictly prevent sharing activities it from happening. is should be stressed that sharing activities is not recommended. Activities should be regarded much like work-items in a personal to-do list.
It's quite common in planning situations, that you have a need to define a task that spawns off more work-items to individual developers, simply because more developers are required in order to complete the task.
But in that case you should see the stream as the task, and the personal work-items as the activities.
Allowing users to share activities would make it unclear when one developer is done and another is taking over. It would also be more difficult to determine exactly when the task is done because no individual developer can be set as responsible for closing the activity - there might always be more developers working on it. Finally the ability to share activities is only possible on shared streams. But imagine a scenario where a task develops and become more and more complex, and therefore - at some point - it is desired to turn the shared stream into a team stream (a development stream that has other development streams coming off from it). The it's not possible, because it would mean that the shared activity approach is discontinued.