Users has to be manually defined, it can't be done through the user interface at this time
Open Trndis config file (~/.config/Trndi.cfg) and add the names, separated by commas:
[users]
names=one,two
Open the registry and find the HKEY_CURRENT_USER\Software\Trndi key.
There, add a string value called users.names. Add the users, separated by comma - for instance one,two
Once the names have been added, Trndi will show a dialog when starting - asking which user to use. You can configure each user in Trndi, start multiple instances and choose a different user for each.
The titlebar will change from Trndi to [username] Trndi for any non-default user. Note that the default user is still available, if you click Cancel.