A reviewer must be nominated for a subset of proposals submitted as part of the Main Call, and for all proposals submitted in response to the Supplemental Call. This is to enable the reviewer to participate in distributed proposal review. For the Main Call, the OT will request that reviewer information be added if the proposal is not a Large Proposal. However, only those proposals confirmed by the JAO to request less than a certain amount of time (currently set at 25 hours of 12-m time) will ultimately be asked to participate in the distributed review. All Main Call proposals over the threshold will be reviewed by panels. More information can be found in the https://almascience.eso.org/proposing/proposers-guideProposer's Guide.
If the reviewer doesn't have a PhD then it is possible to nominated a suitably qualified mentor who will provide support.