Can non-reviewers influence the decision about a paper in a program committee? In a Program Committee (PC), can PC members non assigned to a paper still have a look at it and make comments influencing the decision, assuming that the reviewing system allow them to access the papers and the reviews? 

If they only make a comment, should this comment be sent to the authors (EasyChair, for instance, does not forward the comments to authors)? For instance, consider the case where a PC member thinks that an important reference is missing, regardless of the overall quality of the paper, what should she/he do? Add a comment? Ask to be assigned as a reviewer?
In theoretical computer science conferences (which often use EasyChair), I have seen the following practices:


Once all reviews are in, PC chairs often encourage all PC members to comment on all papers on which they have something useful to say.
Sometimes the comments are just discussion that may influence the final decision in borderline cases.
Sometimes the assigned reviewers modify their reviews based on the comments (e.g., they realised that they overlooked some important point).
Sometimes the PC chairs ask some of the assigned reviewers to incorporate the comments in their reviews.
Sometimes the PC chairs ask non-reviewers to convert their comments to (short) reviews.