For projects intending to host conferences in the coming months, how should the projects handle any potential cancellation fees or deposits previously paid to hotels? Can these costs be charged to the grant? What about fees for attendees?

If the PI (organizer) should confer with his or her program officer and grants management specialist to decide whether to cancel the meeting, postpone the meeting, or convert the meeting to an on-line format. These considerations should take into account any declared states of emergency for the planned location. 

This request for input is consistent with 2 CFR 200.407 which says in part: “the non-Federal entity may seek the prior written approval of … the Federal awarding agency in advance of the incurrence of special or unusual costs.”  Note that any communication to the sponsor must be coordinated through the Office of Research Administration (ORA - Medicine/Nursing) or the Office of Research Support (ORS - Campus).  
If the request is made verbally, written documentation of the decision should follow (an email from the sponsor to the PI and ORA/ORS should be sufficient).  
At many federal agencies, the program officer cannot independently make this type of decision, and grants office concurrence is required.  PIs should follow the requirements for their specific sponsor.  

Duke University has cancelled, postponed or virtualized all university-sponsored events expecting over 50 people in attendance through May 7, 2020, and has urged the transition of all in-person meetings to virtual for the time being. 

Organizers may wish to make a firm decision about rescheduling or cancelling a meeting early enough to enable their participants to take advantage of discounts offered by airlines or hotels related to COVID-19 changes or early cancellation (particularly for hotels). In addition, a decision by the organizer to cancel the conference may assist some participants in convincing their institution to pay their cancellation costs.  

If approved by the sponsor, once cancelled, the non-refundable costs for the venue would be an allowable charge to the project (if the project was scheduled to incur those costs in the first place). In general, the costs for participants should follow the funding stream intended to pay for their attendance; if the conference grant was reimbursing all or part of the participation costs for all attendees, it may be appropriate to also charge some or all of their cancellation fees. If participants were covering their own attendance costs (from sponsored or non-sponsored sources), then then the cancellation costs should follow the sponsor funding their participation or the participant’s employer.