Open your SharePoint site and be sure you have the right permissions to create a Calendar
Create a “normal” calendar so not a Group Calendar in SharePoint and be sure that all members of the team do have permissions to be able to add entries
On the Calendar list – switch to Calendar tab
Choose Connect to Outlook
Within Outlook you will now see two Calendars (the one from SharePoint and your own mailbox)
Open both Calendars in List View , open your own Calendar and select and copy all items over to the SharePoint Calendar.
On the SharePoint site – refresh the page and you will see all items there.
Ask one of the other colleagues to access the SharePoint Calendar URL and let him/her add something and sync the Calendar to their Outlook Mailbox
You will see the Outlook calendar with all the items from the two people and adding entries in either SharePoint or Outlook will update the Calendar