Assigning multiple reviewers to PR

We have a free organization account and we’re trying to add multiple reviewers to a PR but it doesn’t seem to allow us to do this. Every time we try and select another user it just replaces the existing reviewer. However, we have an outside collaborator who has their own organization and they are able to assign multiple reviewers to PR’s just fine.

The only difference I can see between our organization settings and theirs is on the organization settings page - on their settings page it says the name of their organization above the “Account Settings” menu and under the name it says “Organization Account”, with an option next to the name saying “Switch to another account”. On our organization settings page it doesn’t have any of that stuff above the Account Settings menu, which makes me wonder if we somehow have a different type of organization account. However, under “Subscriptions” on both our settings pages it says Github Free.

Any help would be appreciated, thanks!