The whole value would be designate to correspond with how much time and importance relevant to the other tasks of the quarter or semester. I usually consider these efforts as units or tasks of their own but they could just as well be culminating activities of larger units. Your facilities and availability of technology in the school will dictate the size and number of groups. If only one computer, a whole class exercise or spinning off small groups is one way to allow students to use multimedia presentations. Ideally pairs or individuals allows the most freedom.
Time is a never ending problem, there is never enough.
Daily rubrics would be helpful for students to self-assess their contributions. Have enough space and time for groups to isolate themselves is important as this is a noisy process. Allowing others to view and comment on the presentation is important to ensure a higher level of quality and to provide a target audience. Presentation can be to a class, to other classes, or outside groups. All have some value and are useful depending on the subject and content I'm working on at the moment.