Use the search control in Group to look for something shared with that group. Try a word from the moment or note you remember, and use the available filters to narrow the results.
To keep a favourite close by, open a moment and choose Keep this. It appears in the group’s keepsakes for everyone in that group, so it is a shared choice rather than a private bookmark.
Switch to the right group before searching. Search does not make other groups public or give you access to groups you have not joined.