Is it necessary to add instructions, or is it something we expect the user to try ? In this case there's an input where you type an email and add it to a list. Input is only one line, so it doesn't give a hint of the possibility to add multiple emails.
If possible try to have the system be sympathetic to using commas or a space or a semi-colon. All of these are used in different apps they might have already used. If the user enters a space the address can be converted into a tag/label (like stack exchange does). The cursor remains and they can type another. I've found that this is typically understood by users however they need a prod to try it the first time.
I would ensure that any label or intro hinted at the possibility of multiple entries. This could be as simple as saying "Enter the addresses" or "Enter each address". Also have a small example as a previous poster mentioned. If you have room, secondary tip text (as demonstrated in the images below) can help.
Another thing to consider is to have an 'Add' button that hints at the ability to repeat the process. Here are some examples:
Note: the 'Add' button appears within the field when the user gives it focus.
Your question is twofold:
- How can the user know he can enter a list of things
- How can the user know the items can be separated with commas
Let's take an example where you can enter multiple things by separating them with commas:
To answer part 1 of the question: It is the s of "Tags" (i.e. a plural word) that suggests I can enter many things. There is a popup with suggestions that allows me to easier enter values which is a very nice bonus but not fundamental for a user to understand the idea that he can enter more than 1 item.
To answer part 2 of the question: There is nothing that suggests you can use a comma to separate items. However every user seems to know it. It is a number of things that cause this:
It is intuitive: The comma is the most natural character to use, because it is used in language to sum up things in a list.
It is common. Almost every e-mail client, or any text input field that allows to enter a list will accept the comma as a separator.
Trial. Since it is the most natural thing to use, people will try to use it. As soon as they see it works, they will continue to use it.
I would suggest adding something like contextual help tool tip icon next to the input/input label that the user can click. This way interested users can choice to see the extra instructions and the front end label can stay short.
Additionally I would suggest adding support on the back-end of your application to the comma in case a user doesn't realize that they can do that. I wouldn't let the form error out if the commas are missing as this would be more frustration to a user if they have to return to the input to add a few columns. See example below:
According to me, the best way out for this would be to put an example just under that
text-box, in tiny (but readable) font, explaining user how to comma-separate multiple emails.
If its a mobile screen, you could try putting such
Why do so: An example like this doesn't require user to go for an extra click on something like an information bulb or a
? and then read text instruction. I feel people find it more intuitive to see an example of what has to be done. And if you're giving it without any extra effort, its better.