This sentence is in
hypothetical subjunctive mood. This mood indicates unlikely or unreal conditions and
always uses "were" as "to be" form.
e.g. If I were a rich man, I would buy my own Ferrari.Here I am just talking about things I wish would happen or talking about what would happen if things were different from what they actually are -> hence hypothetical.
By far the most common use of the hypothetical subjunctive is the use of the subjunctive after
"if" clauses that describes a hypothetical situation.
In the example given by you. The correct wording should be:
We could start the meeting if SAM were here.This situation is hypothetical because SAM is not present at the meeting.