For user invitations: self-hosted Grist doesn’t yet have built in email templates, this is a TODO (some notes at feature request: smtp settings for self hosted installation · Issue #146 · gristlabs/grist-core · GitHub).
To limit what guests can do: you could change GRIST_SINGLE_ORG=docs to anything else but docs such as bibo. When set to docs, Grist will work like docs.getgrist.com and allow people to try out Grist anonymously, which I agree doesn’t make sense for your server. When set to something else, such as bibo, Grist will create a bibo “team site” and grant access to the user with email GRIST_DEFAULT_EMAIL. That user can then grant access to others, including optionally the general public (but the public won’t have access by default).
Before changing GRIST_SINGLE_ORG, be aware that you afterwards you won’t be able to find any existing docs via the website. One option is to download them first, then re-upload afterwards.
Not sure why your container crashed, everything in the log you pasted is innocuous looking GETs.
The default worked for me. If you’re having trouble: generally, Grist identifies users by email.