By default you can open a colleagues calendar however it will only show free/busy information. In order for colleagues to view detail you will need to edit the permissions for your calendar via Outlook.
Select your Calendar
Along the top ribbon select Calendar Permissions
You can Add your colleague from the address book and then select the appropriate level of permissions from the presets or choose custom options
Alternatively you can select the Share Calendar button from the ribbon
Select your colleague in the To... field and then choose the sharing leevel next to Details and select Send. With this method you can choose more than one colleague to send the sharing invite too