I wont boot someone from a fireteam because of a low grimoire (under 2700), but im gonna keep an eye on em. Excessive dying, hindering progress, will get you booted. If you show up and say "ive never done this raid/poe before" then ill stick with you and we'll get it done. Grimoire doesn't seem to actually indicate skill level at all. Just experience. A lower grimoire player may be really good at pvp and good at pve combat, but experience makes a big difference. Keeping the round alive with situational awareness in critical moments is invaluable. That usually comes from experience.