You should think this through carefully so that no one can game the system. This could easily just reward popular students. Many nice students will not want to grade people down (making the grades basically meaningless), while a few hyper-competitive students might give everyone else zeros thinking that it raises their own grade.
Scarcity of votes could force people to make thoughtful votes.
I've heard when you ask people to grade themselves and their groupmates, you can get truer evaluations by giving them a set of points to split. E.g., 3 people in group means divide 30 points among you. Instead of the usual, "Oh, everyone gets 100%!" students are often more realistic if boosting one person's score without merit takes away from another person's score. (In any case, you might not want to bind yourself to using the students' scoring directly for grades, but rather using it to inform your decision.)
You could apply this "scarcity of votes" in a couple of ways.
Come up with, say, 3 rubric categories (content, oral presentation, visual presentation). After each presentation, ask people to vote (hands or clickers or some web poll app) on which category the team did best. Two teams that were great overall or horrible overall may the same score as each other, but each gets feedback about which part is most productive to spend time improving (the rubric category they got the fewest votes on).
Have all the voting at the end of the session. Write each group's title on the board (or project it) and give each person 3 sticky notes place on the board next to the titles, to vote for their 3 favorites.
If voting happens after each presentation, I'll also note there will be strong ordering effects from non-trained evaluators. If all the topics are similar, the early ones will have high scores and the later ones will have low scores. If it's a harder task than expected, then people will be kinder to later presenters. These effects may still be present if you save the voting until the end, but perhaps not as strongly. (Possibly you'll get recency effects, that people will remember the most recent good presentation and vote for that.