There's not a difference between the trial version and the paid version. The only thing is that you have a period of time where you don't have to pay yet while you test it out.
So, you're trying to add users to groups? Are they already on the instance itself? Are you able to send us any screenshots so we can see what you're trying to do exactly?
I ask, because earlier you said you were trying to invite users and later you also mentioned adding users to groups.
These are three separate actions. Please clarify with us exactly what you need to do. Providing a screenshot will help.
If you need to share a page, once you start typing it checks the database against your existing licensed Confluence users. If it doesn’t match then you can still type in an email address.
However, if the page you’re trying to share isn’t public then the person you’re sharing with won’t be able to view the page. That’s just something to keep in mind.
If you continue to have issues please feel free to contact support and they can help you out.