I participated in the competition from the group, but accidentally left the group and now I can't find her, to continue the competition, help me find the group where the competition was
If you get some email notifications from Garmin, then check your email archive, there may a notification from the time when you joined the group, or when some of the group challenges were launched or completed. Also click the Notifications icon in Garmin Connect Web and scroll down in the list to earlier dates, there may be perhaps old notifications about group challenges, events, etc. Another possibility is requesting the full dump of your user data at https://www.garmin.com/en-US/account/datamanagement/exportdata - the information about the group might be hidden somewhere there, although analyzing it won't be easy for an inexperienced user.