There are some specific sneaking quests which will fail if the sneak skill isn't high enough, sneaking in the old ruins is a good example.
Many skills automatically enable *across* the group, haggling for example, where even the party member with no haggling skill still shows the % haggling of the highest skilled haggler on the trading screen!
Sneaking however, is not spread group-wise but is individual - and because you cannot separate the group members, sneaking appears difficult at first try.
While i was struggling trying to sneak during the above mentioned quest, very appropriately while loading a saved game, a hint popped up which read as follows...
Can't sneak? - "a sneaking party is only as proficient as the group member with the lowest talent prowess value"
So i bumped up the party members which had low or no sneak skills and sure enough the whole group sneaked passed all the cultists and guards without any problem.