Access & roles
Invite somebody from the Members page. They get an email, and they gain access once they accept it and verify their email address — inviting alone gives nobody access.
Pick the role at the point of invitation. Each role's description explains what it can do; when in doubt choose the narrower one, since changing it later takes seconds.
When somebody leaves
Remove them from the Members page. Access stops immediately. Their user account continues to exist — they simply no longer belong to your organisation — and everything they created stays exactly as it was.
Do not share logins
A shared account makes the activity log meaningless: every change is attributed to whoever the account belongs to. Give people their own invitation.