How does the help action 5e work in d&d?

Suppose you take the Help action 5e. The creature you aid gains an advantage on the next ability check it makes to perform the task you are helping with, provided that it makes the check before the start of your next turn. Alternatively, you can aid a friendly creature in attacking a creature within 5 feet of you.

Is help an action or bonus action?

In dnd combat, one teammate may ‘Help’ another willing ally:

Help: You may lend your aid to another creature in the completion of a task. Alternatively, you can aid a friendly animal in attacking a creature within 5 feet of you. You feint, distract the target, or in some other way team up to make your ally’s attack more effective. If your partner attacks the target before your next turn, the first attack roll is made with an advantage.

If something can be performed using a bonus action, it is always explicitly stated in the description. The lack of such a statement indicates that it can only be used as an action.

Concentration is only broken by specific things, which are listed on page 203 of the PHB and are:

It is casting another spell that requires concentration. Readying an attack or casting a spell with a casting time over 1 action also requires concentration (PHB 193 and 202, respectively).

Taking damage

Being incapacitated or killed

Could you use the 5e Help action outside of combat in dnd?

Helping someone outside of combat is under the header “Working Together”. Yes, you may help another player to give them an advantage outside of combat.

PHB 175 states: Sometimes two or more characters team up to attempt a task. The character 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).

Being able to help someone outside of combat is still up to the DM’s judgment, however. It’s limited to situations where a helper would be useful and can accomplish the task independently.

What can an Invisible PC do to take the “Help” action and remain invisible?

An imp’s invisibility does not end when taking the Help action

The Help action 5e states: ‘Alternatively, you can aid a friendly creature in attacking a creature within 5 feet of you. You feint, distract the target, or in some other way team up to make your ally’s attack more effective. If your ally attacks the target before your next turn, the first attack roll is made with an advantage’.

Note that there nothing says an ally is required to see the helper, nor that the helper be able to see their ally. Thus, an invisible familiar can take the Help action. The question, then, is whether or not the Help action ends the Imp’s invisibility.

Now let us see what The Invisibility feature states.

The Imp magically turns invisible until it attacks or its concentration ends (as if concentrating on a spell). Any equipment the Imp wears or carries is hidden with it.

Taking the Help action in dnd 5e is not an attack, as it does not involve an attack roll whatsoever. Again, no part of the Help action describes the Help action as an attack. However, ask your GM to make a very similar ruling to the contrary.