Once you've created your group you'll want to invite other people to join. To do so, go to the Members tab under Group Settings and click Send More Invitations. You'll see a display similar to this one:
You can enter a comma-separated list of members to invite, along with an optional personalized message. If you don't know whether the people you want to invite already have Zotero accounts, you can enter their email addresses; each invitee will receive an email with a link inviting them to set up a Zotero account and join your new group.
Displayed at the bottom of the Members tab of the Group Settings is a list of all members who have been invited but who have not yet responded. You can cancel invitations that have not yet received a response.
Note that currently, invitees must use the login link in the invitation email to log in or create a Zotero account, in order for the invitation to be accepted. If an invitee decides to set up an account without clicking the link, he/she will have effectively joined the group without accepting the invitation. In this case you may end up with spurious unaccepted invitations in your group's member settings; these can be safely cancelled without affecting group membership.