To assign a particular abstract to a specific reviewer.
Disable the Automatic Allocation of reviews in Event » Abstract » Setup » Review Stages » Edit
Select ‘Disable Automatic Allocation’
In Program » Abstracts/Search search for the appropriate abstracts. Tick those you wish to allocate to a reviewer and select add a pending review to abstract in the dropdown box at the bottom.
Hit go, select appropriate stage and then select which reviewer/s you wish to have review the abstracts. Then preview, tick Confirm and Process.