Many research conferences hold an in-person program committee meeting for all of the members of the program committee, where the PC members make the final decisions about which papers to accept or reject. (At least, that's how it tends to work in computer science; I don't know about other fields.)
Can program committee members use federal grants to pay for their travel costs to attend a program committee meeting, for conferences where they are on the program committee? Generally speaking, what funding sources do professors tend to use, to pay for travel to these meetings?
For each grant, or type of grant, there are rules specifying what it can be used for. You should follow these rules and, unless you ask us about a specific funding program or agency, it's impossible to give more specific advice.