No, they cannot
There are 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 is saving each of your versions of the agenda.
Therefore, we 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 write content etc.