You need permissions to access those calendars (emails in dag-groop@googlegroups.com have the permission). Add the calendars via “Other calendars > Add by URL” (copy the links below into URL entry form).

https://calendar.google.com/calendar/ical/fc7tke90gqp3gs1p3ifbk20f3c%40group.calendar.google.com/private-a5ac8271a2da3700491fefa8db6cf8e3/basic.ics

https://calendar.google.com/calendar/ical/v2kf6ugc7ocsdqsm4reg2teu2k%40group.calendar.google.com/private-fde0bc92e6d2118b5eecc6613cb9f6c1/basic.ics

https://calendar.google.com/calendar/ical/dcuiu6e2gfnqv3rq85crg5qpok%40group.calendar.google.com/private-a921f0f354e1579c9d5749ae3cce39bc/basic.ics