There are no public directories of Guild members or guilds. No-one can ‘look up’ a group or individual.
All groups are invitation-only and completely discrete. Only the people in your guild(s) can see your messages/threads and profile.
Direct messages are only visible to the two participants.
Legally, you own all of your data and contributions: Guild is merely storing it for you and you can request it or delete it at any time. Guild is GDPR-compliant and data is stored in EU data centres only.