Is that possible to use the # of a project if you didn't created it?

For example I created #thrdr and we are 2 working on it, but it seems that this other person cannot include it in its tasks

Looks like that's not possible for now:

Ben is correct. Right now the best approach would be for both makers to create the same product with the same hashtag. Using the same hashtag will make things easier to merge them down the road.