NPCs are usually Non-player characters. In this game they are family members of the students, or other people involved in their lives. They don't need a full application (though they do need a little info) and can be played as part of the character's journal (or you can have another player play them to help you with story). They won't have their own tag on the community or be an integral part of big plots, but they can help round out your characters story lines.
What I would like from you:
Relation to character:
Who they'd interact with:
Are they alumni? If so, house and year they graduated.
Any other info you want to share: Like their job, are they a muggle/squib/wizard, anything else you might think is important.
PB: Check your preferred pb against this list
NPCs will be listed in the comments.