Misleading search results in new user enrolment control

In the new "enrol users" control, the search results can be very misleading.

Firstly, it's not particularly clear that there's a 30 record limit.

Secondly (and probably more importantly), the 30 record limit doesn't take account of users already selected.

To reproduce the problem:

1. Create forty users: Person One (user1@example.com) etc
2. Go into participants in a course and click "enrol users"
3. Search for example.com
4. You see the first 30 users. Select them all.
5. Search again for example.com - you see a message saying "no suggestions" so you think you have enrolled everyone from example.com, but there are still 10 users who are never shown for that search (even if you search for something else and go back to it)

I realise this may seem like a very contrived example, but it's common in universities for departments to run a course for a group of people in a partner organisation. In that case they'll generally search by the organisation's email domain rather than for each person individually.