To allow multiple people to connect their calendars, first they'll need a user account. Head to Manage Users and create a new user for each person who would like to connect. For the Access Level, select "View & Edit".
Also, be sure to give each new user access to the Acuity calendars they should be able to sync with:
Once the new user account has been created, they can log in to Acuity and connect their own calendar accounts and set up syncing under Business Settings - Sync with Other Calendars:
*Note: Changing your password in Google will revoke Acuity's access to your Google calendar. If the password is reset in Google for the calendar you're syncing to in Acuity, then the sync must be re-enabled in the Business Settings > Sync with Other Calendars page.