In the section Working Together, the basic rules state:
Sometimes two or more characters team up to attempt a task. The character who's leading the effort — or the one with the highest ability modifier — can make an ability check with advantage, reflecting the help provided by the other characters. In combat, this requires the Help action (see chapter 9, “Combat”).
A character can only provide help if the task is one that he or she could attempt alone. For example, trying to open a lock requires proficiency with thieves' tools, so a character who lacks that proficiency can't help another character in that task. Moreover, a character can help only when two or more individuals working together would actually be productive. Some tasks, such as threading a needle, are no easier with help.
I understand that this happens when the group can, and will work together to do something, but I'm wondering about the limitations of these definitions.
For instance, in a game, our characters had to climb horizontally a porous wall for ~150 feet. Below that wall, 100 feet of void then the sea. Some characters were reluctant doing this as their athletics skill is the lowest, going in the negatives. The other characters convinced them to do it anyway, arguing they'd help. So the characters all attached themselves to a rope and started the climb. The help promised by the other characters was under the form "put your foot here", "put your hands there", "pull your weight in your hands" and other advises like that. Individual tests were used all the time. After 50 feet, the whole party was swimming in the sea.
After the session, I was wondering whether I should have asked the DM if a group check should have been done. Our DM never ever uses group checks as defined in the quoted part, and I'm wondering whether he knows about them. Given that we've had one such situation roughly every two sessions (we always failed those situations), that's definitely something I'll ask him about.
But nonetheless, I feel this is a thin line as no explanation exist about what help is (instructions, physical help, etc.). If more able characters provide instructions, are those help? How is help defined, and when does an individual ability check become a group ability check?