Invite people from Settings → People: click + on the top right, type their email address and click Invite. They get an email and sign in with Google or a password of their own. Then give them roles on the Teams & Roles page, so everyone knows what they own. Owners and admins can invite people.
Add someone without inviting them. Someone who should hold a role but doesn't need to log in, such as a volunteer or a freelancer? Open the role, type their full email address in the person picker and choose Add without inviting. They get no email and no login. Choose Invite {email} and assign to do both at once.
Using Google Workspace? Connect it in Settings → Integrations to bring in your colleagues from your Google directory. Switch on Auto-sync to keep the list up to date every day. See Members & guests for changing someone's access or details.
Settings, People: click + on the top right, type an email address and press Invite