If you can do OLCI, no one need ever show up at the check in counter. This assumes that no check of travel documents--for example, to see a visa or the passport expiration date--is necessary.

You do need to show up with photo ID to check bags. I would assume that it would be a problem to check a bag on someone else's ticket (for example, because they get a free bag, but you cannot check an additional bag without paying). If there's a companion bag fee waiver (for instance, due to one person's elite status or credit card), often the rule says same PNR, but this might not be enforced if all appear to check in together.