Hey Andrew!
We've made a bunch of improvements to the 'Connection' page since the release. The one relevant to your question was Bug 70580. We also expanded the options for Windows authentication with Bugs 64719 and 64894. The next release of WebLink 10 will have all the same functionality as WL 9 as far as login methods and UI conveniences.
In the meantime, you can work around the limitations in the UI by making some small changes to the configuration file on the server. It'll be in [installation directory]\Config\WLConfig.xml.
In the UI, set up a repository with one of the users' info. In the relevant <VirtualDirectory> node, find the relevant <LoginEntry> node, for example:
<LoginEntry DisplayName="MyRepository" Server="servername" Database="MyRepository" DBID="0" Password="encryptedpass" Username="publicuser1" AutoLogin="1"/>
For each additional user you want to set up, you'll need a <LoginEntry> with the additional configuration. The configuration will look exactly the same as that one, except with a different DBID, username, and password.
The slightly tricky part here is that the password is encrypted, and you need to go through our UI to calculate that value. So instead of immediately copying and pasting to create your new node, you'll have to save that XML in a different file, set up publicuser2 in its place in the UI, and then copy the XML for publicuser1 back over to the file. Again, remember to change the DBID to an unused one.
Sorry for the inconvenience in the current release! Look forward to next release, when you can do this in 5 seconds in the UI instead of needing to go into that file. :D