Scheduling a matrix type project

[Antonio]

I am scheduling a project where I have people resources that will only
work on the project %20 of their time. In the task assignments I
assign different tasks to them for 20% of their time. Whenever they
have a meeting, I assign 100% of their time to the specific hours for
that meeting.

The problem that I am having is that Microsoft Project is not
restricting the resource by saying that they can ONLY work 20% of their
time on this project so if there are tasks that add up to greater than
20% of their time then they will be delayed.

Is there any easy way to do this in Project? Do I have to individually
change the percentage of work that they are doing to fit the overall
20%?

Thanks.