Inviting people
Go to Team → Invite, enter the email address and pick a role. The invitation link is valid for a limited time and can be resent or cancelled.
Roles
Each workspace has owner, admin, manager, executive, member and viewer roles. Owners can customise what managers, executives, members and viewers can do under Team → Roles.
Two-factor authentication
Each person can turn on two-factor sign-in under My account. Owners can require it for everyone in Settings → Preferences.
When someone leaves
Remove them from Team. Their open tasks, leads and opportunities are unassigned so nothing is silently lost, and their automation rules are switched off.