Like many have said... its just totally dependent on the group. Most of the times (in solB at least) when I have had members just randomly leave to sell, it was a pain and it did halt the xp. The group eventually just asked the person to hang around because we mostly wanted xp and it was hurting our groups potential and they were fine with it.
In a similar event, I have been in real long groups where we would let people afk for a longer period of time and would hold the spot. The groups had been together such a long time and we knew they would keep on going. However, some people did have to tend to basic needs.... food/shower/ect.
[You must be logged in to view images. Log in or Register.] These cases, in addition, also had limited pulls in the camp (Oggok guards for example). So one person leaving really did not hinder us to much.