Hello Juha,
user creation during a share process should not result database corruption, I've tested it and there was no problem with that.
We've recently fixed a bug related to the user database, I think that might have happened on your server as well, so I think it was only a coincidence that it happened during share. The upcoming Hotfix (2) will contain that fix.
Regards,
Marton
Marton Kiss
Chief Product Officer
GRAPHISOFT