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
If you want to set permissions for everyone who isn't specifically specified, you can change the 'Default' permission level.