Predictive/autosuggest search and results

In our web application, we have a search field for users to search for other users or other groups of users to add to their project.

Both users and user groups can be a long list. The search field is within a form, so the selection of items must be within that interface. There is no dedicated search results page. We are planning on separating the search dropdown into two sections, users and user groups. Users add items by clicking "add."

Question: Should we use scrollbars to convey to users that there are more results or trust that users know they need to continue typing to filter further the results?

TIA --Mor