If you're racing on the track, it's not unusual that gaps appear when the peleton is lined out. If you're in one of the gapped lines, chasing like mad to get back to the leaders, it's quite common to yell at the guy leading the gapped group (and moving a little too slowly) to "swing up!" After all, you don't want to go around the guy, because you have to around the outside, and he just might swing up into you, plus it's more effort to pass a rider on the outside of a turn. By the same token, if there's a one- or two-bike-length gap in front of you, and a rider has just swung off and is coming back down the pace line, you might yell at him to drop "IN!" right in front of you, effectively closing that gap. You have to communicate with other riders, else you get dropped.
On a casual training ride on the road, things aren't quite as intense. If someone at the front is going too slow for your liking, there's usually room to pass him on either side. Usually you'd go opposite the side riders have been swinging off. If it's an intelligent/experienced pace line, this will mean passing him on the leeward side (you always swing off into the wind). You probably don't want to yell at the guy to swing off simply because there might be a car back, passing the line, and he just might swing off without looking... Once you pass him, he'll get the message.
That said, there's nothing wrong with telling the guy in front to "pick it up!" If he can't, he'll prepare to swing off, making sure there's no car coming up.
If it's a club ride, pace lines are expected. You don't have to ask. If some club passes you, it's OK to jump on the back, but I would just draft, allowing guys coming back to slot in ahead of me. If they want you to do some work, the guy will leave that gap open, forcing you to close it. Now you're in the pace line. In Paris-Brest-Paris, the Seattle Randonneurs had their own pace line going, and they didn't want anyone else in the rotation. They had a "ticket puncher" in the back who made sure that non-SR riders did not enter the rotation. I just sat behind the ticket puncher; fine with me, I didn't have to do any work!
Luis