Colleague demanding to add additional participant to funded grant Recently I had a moderate sized grant funded from a national agency.  The grant aim was to obtain data to combat a new disease causing significant issues in corn.  As you may guess, the new, high profile disease has resulted in many interested researchers, with very limited funding opportunities.  One of the reasons I wrote this grant was to pool expertise and resources between labs and states in an effort to more rapidly address this disease issue.  However, two of the individuals on the grant have been less than cooperative.  One has refused to follow the guidelines set in the grant and share results and progress with the group, and another blatantly made efforts to scoop research that was designated for my lab in the proposal. Recently, individual 1 contacted me, demanding I include his colleague at his institution on the grant.  However, this person is redundant,  and we already have an excellent individual leading this research area in the proposal.

How should I address these issues?  Part of me wants to be direct with these tenured colleagues, while another part is tempted to simply roll with the punches.  However, being an assistant professor, it is not necessarily beneficial for me to allow others to take advantage of my ideas,  efforts, etc.  we are told to be collaborative and write multi PI grants, yet come time for promotion, being middle author on a bunch of papers because I allowed others to roll over on me may be problematic.
The grant has been awarded to you, so you are responsible for making decisions on how to allocate the funding. If a bad choice results in less success or output, this will reflect badly on you much more than it will on other grant participants. So if you do not consider that adding this person will positively contribute to the results, you should not add them.

As others mentioned in the comments, you should discuss this with your department chair, and make sure that you clearly state your opinion and give enough reasons. Your chair might have additional motivations and it could happen that they want to add this person for different reasons (which would make your situation more complicated), but likely (hopefully) they will support you in your decision.