This post is free advice for grad students: just say no. As you progress through grad school, you'll probably be asked to do more and more stuff. A committee here, a book chapter there, and a million meetings and emails everywhere. In the interest of your sanity, learn to say no. A lot.
This is something I've failed to do over the past 4 years, and I've often paid the price for it. And I've said yes to stuff a lot of the time because it was nice to be asked. "Wow," I thought, "people want to work with me!" It's flattering. But you should only answer yes to these requests if it checks two boxes: Does this help you graduate in a timely manner? Does it help you get a job?
Lest I be accused of advising grad students to do the least amount of work possible, this is not what I'm suggesting. My own biases suggest that success often means doing more than just your coursework, graduate assistantship, and dissertation. But you have to be selective. If you want to go into academia, being the grad student representative on a selection committee is probably a great idea. If you want to go into the policy world then writing that monograph for a think tank is probably a great idea. If you have a family then that job outside of school is probably necessary.
Here's what you should probably say no to. Conference papers that aren't written as part of your fellowship or dissertation. Probably most book chapters and encyclopedia entries. Book projects that aren't your dissertation. Be selective about guest lectures, article reviews, workshops, and requests for random writing projects. You will probably have to make a list ready in your head of the people you will always say yes to no matter the circumstances. Everything else needs to be weighed carefully - your default should be no, unless a solid case can be made that a new commitment is worth your limited time. There is no one answer for every person - we all have different goals and needs, but have a mental rubric to help you navigate these decisions.
Time is a scarce commodity that becomes more dear during grad school. Over-extending yourself is easy to do and it can make it hard to complete the things that you absolutely have to get done. Like your dissertation.*
*In case any of my committee members read this, I'm still on schedule!