Open Who and pick a group, a past event or a recent person, or type a name. Everyone you add is listed under Selected.
For someone new, type their name and a mobile number or an email address. Either one is enough; you do not need both. Someone with only a mobile gets their invite from you rather than from us: see Guests with no email address.
Tick a group to add everyone in it, or open it to pick some of them. Tick a past event to invite the same people again; it shows how many came last time.
Add multiple, at the foot of the list, takes many people at once: one per line, typed or pasted from a group text, a note or a spreadsheet column. Each line shows what we read from it before anything is added.
A couple or a family who come together are one invite: add the second name with Add to party. Households, plus-ones and kids explains how they count.
Your People page holds everyone you have invited, and adds people without an event too: see Your people.