How can I check for availability across a number of accounts?

If using scheduler, you use the scheduling_method with round robin to check across all calendars availability, see Scheduler Smart Integration.

In order to add additional calendars for availability you will need to add their accounts, as such will need tokens.

You can do this without scheduler simply by querying  the /availability endpoint using the participants email addresses.

Further Reading:

https://developer.nylas.com/docs/user-experience/scheduler/schedule-editor/#smart-integration 

https://developer.nylas.com/docs/api#post/calendars/availability