However the presence or absence of a comma is not a completely reliable criterion. Lots of writers leave it out when it should be present; and in some cases a comma is used before a restrictive relative clause for various syntactic reasons.
In spoken language, it is easier to make the distinction: a restrictive relative clause shares the same tone unit as its antecedent; a non-restrictive one opens a new tone unit. The listener will perceive this, if only on a sub-conscious level.
For the sentence you give, the context may (or may not) make it clear. If we learn from the context that the ABC organisation has just over 150 members, then we can conclude that we are dealing with a non-restrictive clause: all its members will be acting as volunteers.
If on the other hand, we learn from the context that the ABC has considerably more than 150 members, then we are forced to conclude that the relative clause is restrictive: only about 150 of the, say, 300 members will be acting as volunteers.
In conclusion, if the context helps you to make the distinction, fine! If it doesn't, the chances are that it's non-restrictive because of the comma preceding it.*
*MALAN, N., La proposition relative en anglais contemporain. Une approche pragmatique. Gap/Paris, Ophrys, 1999.
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