Oh, sorry, I didn't catch the distinction you were aiming for.
I think in the example you've given below there are a couple of things to consider:
One is, I think we need to be careful laying the most distinct boundaries around the two categories to the exclusion of not being able to take advantage of providence in sharing the Gospel message.
But that being said, I think there is also the danger in men taking it upon themselves to be "a church of one" and invest themselves with whatever authority they feel is necessary to "get the job done." If a man has not been ordained and commissioned by a local church body/governing body, then in MANY circumstances, he probably does not possess the qualifications to rightly divide the Word, nor shepherd people's souls in a way that will not prove dangerous to them.
In your example, should a person not seize the opportunity, if he has a Gospel message on his heart, to share it with a group who may be willing listeners? Why restrain oneself should one be ready, willing, AND able?
I think the distinction might come into play when a man decides that ON A REGULAR BASIS this will be his "calling." At that point, it would seem he has moved past "witnessing" and become a self-proclaimed "preacher", with no authority, no accountability, and who knows what other deficit. This would seem to be a prideful stance and deny the authority of the Church.