If you want users with email addresses on one domain to book room resources on a domain in a different M365 tenant you need to allow this in Exchange online. There are several articles on the Internet explaining this process (for example here). Ultimately, you need to make sure that rooms on domain A are bookable by users on domain B from within Outlook before this will work in RICOH Spaces.

Once you have configured Exchange to allow all the required users to book the correct rooms from Outlook you can then begin to set up RICOH Spaces to do the same.

Step 1: You will need to add an Integration within RICOH Spaces for each M365 tenant that contains a room resource (see this article).

Step 2: For each room you will need to select the correct integration for that room's calendar (see this article).

Step 3: In RICOH Spaces you will need to switch your Calendar Management and Permissions to "User Token." To do this head to Organisation Settings > Global Policies (you will need to be a Global Administrator in RICOH Spaces to access this option):

Before following the steps in this article please ensure that from Outlook/Exchange Online the rooms that you want be bookable can be booked by the user accounts that you need to book them. Without the correct M365 configuration in place, switching to User Token permissions will not be successful:

If you want users on different tenants to book rooms hosted on multiple tenants then the M365 configuration is needed on every M365 tenant that hosts a Meeting Room Resource:

Only once all the M365 tenants that host rooms have been successfully configured will switching to User Token permissions work as expected: