How to allow B2B users in Microsoft Places workgroups?

Our company recently purchased another company. While we are preparing migrating users from the two legacy tenants to a new, third tenant, we collaborate with each other using Cross Tenant Synch and Cross Tenant Access. 

I am currently testing out Places, but I am unable to add my team members from the other company to a Workgroup in Places: "1 user(s) couldn't be added, the administrator has disabled adding these user(s) to any workgroup". 

Any idea how/where I can change this? I am Global Admin.