Can a character provide help with a Persuasion check?

Question

In page 175 of the player's handbook under section Working Together it reads:

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.

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.

Anyone can attempt to persuade a character alone so the first requirement is fulfilled, but would multiple individuals working together to persuade someone be productive?

Answer 1

Yes they can

Anyone can attempt to persuade somebody alone, even if they are not proficient in Persuasion, so anyone can help with the check.

There are also ways one can think of how a second participant can make the persuasion more productive. For example, they can give subtle signs to the speaker if they are going in the wrong direction (the classical "kick under the table"), they can bring up secondary supporting arguments, they can confirm that what the speaker says makes sense etc.

Social Proof is one of six well-established¹ methods for persuasion: when more other people seem to believe something, it is harder to not accept it.

You as the DM might ask the player how his character is helping, what they are saying to improve the attempt, to allow them to help. For social checks there is always some tension when you ask for "playing it out", because that is not required for example for physical checks like Athletics, and so demanding it shouldn't disadvantage players that picked social skill proficiencies vs players who picked others. However, when you allow anyone to try this, there is no such specific investment of character development resources needed, and asking for some creative input should be fine.

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¹ Robert Cialdini, Influence: The Psychology of Persuasion

Answer 2

"How do you do that?"

would multiple individuals working together to persuade someone be productive?

The rules do not describe this. How it will work is up to the DM.

The play of the 5e game goes according to this basic pattern.

1. The DM describes the environment.
2. The players describe what they want to do.
3. The DM narrates the results of the adventurers’ actions.

The DM can (but does not have to) ask for ability checks before they narrates the results. When a PC tries to persuade a creature, while another PC is trying to help, the DM might ask for an ability check (with or without advantage), or just say what happens. The DM often asks follow-up question, like "How do you do that"?

This is different from the 3.x edition, where RAW it goes like that:

PC1: "I want to use my Diplomacy skill".
PC2: "I want to aid".
DM: "Okay, make a Diplomacy check with +2 bonus".

This is not how it works in 5e though. See How to use skills — did this change between editions and how? DM is supposed to use common sense when figuring out outcomes. Working together when persuading someone seems completely plausible to me, however, it might depend on a particular situation.