Everyone who works your leads should have their own login, so replies and activity are attributed correctly and you can control who sees what.
To add someone: go to Settings then My Staff (or Team) and add a member with their name and email. They'll get an invite to set their own password. Assign them a role or permission level at the same time.
Setting the right access: front-desk or reception staff usually need Conversations and Calendars but not billing or settings; managers might need reporting; owners keep full access. Limiting access keeps things tidy and secure as the team grows.
Seats: your plan includes team seats for your staff, and higher plans include more. If you need extra, you can add seats anytime from your billing settings.
Tip: assign conversations and calendars to specific team members so leads route to the right person automatically, and so your reporting shows who's booking what.