No, they can't.
There is a lot of things that can go wrong, if you are more than one person working on the same agenda. You could risk losing the work you have just made, when each of you are saving each of your versions of the agenda.
We therefore recommend that there is only one person at a time, building an agenda. When you move away from this agenda, there is no problem for another person to work on it. The main thing is that there is only one person at a time to create items and writing content etc.