well, only way to 'grief' behest, is when lvl 50 person signs up , refuses party, and kills all mobs alone., howevcer, you can report such behavior, and get the asshat baned <_<. However level sync is a good idea i really liked it in games it was included, especially right after i joined, i allowed me to play with the guild, nbo matr if those were my low level quest, or their high level ones.

Excelent example was City of Heroes, with Sidekick/exemplar system. Sidekick, allowed low level person to get temporary boost to level of the mentor, however he could use only skills from his real level, and got exp, as he would be killing monsters of his own level. Exemplar worked other way, allowing high level player to lower himself to someones level, he is not gaining exp, just paing debt ( death penalty ) or its converted to influence (ingame cash)