What is the best way to store a task for multiple users in TFS 2010 ? I can only assign one for one task.
(for example: when I plan a demo to all developers)
(this is a scrum Msf Agile project, where the task is part of a user story)
I'm sorry to tell you that you can't assign multiple users to a single work item out of the box; At the same time, I do not recommended trying as this, as it does not fit the model in TFS. The conventional / recommended way to handle this type of scenario is to create multiple tasks; one for each developer in this case. You can easily accomplish this by copying a set of tasks using MS Excel. Another option (given the example you used) is to create a "Meeting" work item that has multiple drop-downs - one for each person that would attend meetings like for a demo or a technical review.
Yet another option is to create a custom control to format and store a list of users. This would likely be relatively complex to maintain, as you have to distribute it to each user's machine (it will need to be installed locally), and last I checked you would need 2 versions; one for the Team Explorer user interface and another for the Web Access tool that most people use to create work items from a web page on their TFS server. Future updates to TFS could possibly break your custom control. It is rarely worth the effort. Another downside is the you would likely be limited by how you can use MS Excel to work with the data you store in the field that the custom control works with. If you want to look into this further you can find some examples in the following CodePlex project: http://witcustomcontrols.codeplex.com/
You might consider your true goals in tracking such things as meetings and other items you want to assign multiple people to. Tasks are the heart of tracking progress of user stories in the MSF Agile Template. Tracking meeting attendance does not typically relate directly to a User Story, for example; so it won't typically assist you to determine how much close you are to being "done" with a User Story. If you want to take advantage of the existing reports, then you should organize your tasks so that they roll up as child work items to User Story (or Bug) work items.
Short story: you can't. Work items in the Process Template of Microsoft are designed to target nobody or only one User.
Now you can customize the Process Template to change this. Take this post for instance, the customization works for group. But I don't recommend you to do so because TFS is basically not designed for that and you may end up disappointed.