A collective is a coercive group of individuals, where the coercion is turned against the individuals who are members of the group. Collectives do not rely on unanimous consent, but on "majority rule".
A voluntary group can still do things collectively as long as there is unanimous consent.
A voluntary, ad hoc group is not something I would consider a "collective", even if they employ coercion against individuals who are not members of their group. A group can be purely voluntary and still be doing the wrong thing, but a coercive group is incapable of doing the right thing.