Dynamic weight paint help

I was wondering if it was possible to have a brush affect a vertex group only.

Take your time with setting your vertex groups, Automatic weights, envelopes and groups is a good(ish) way to start, Just make sure you edit your bones first to adjust their settings first and expect go back to manually add and remove verts from groups.

Im talking about dynamic paint. when using another object as brush, how can i have it add weight to the vertex group only.