Pretty monumental decision needs to be made sooner rather than later, and I understand there are pros and cons to each.
There are a total of 5 companies that are all part of the same family of companies, with one of the 5 companies being the mothership/holding company.
To facilitate collaboration (Teams) I've initially thought that creating everyone inside of one tenant is the better answer.
However, if these companies, as they grow, institute their own policies for things, would it make more sense to break them out now into their own tenant?
I'm also thinking of branding situations in Azure AD and internal company communications may be better achieved with individual tenants, but would collaboration and granular permissions suffer as a result?
Want to make this transition from G Suite to Office 365 a great move; one that everyone is glad happened and not one that is regretted. Any advice, best practices, etc. are welcome and appreciated.
The answer is completely depending on the amount of collaboration between the companies. As every company has their own tenant collaboration becomes harder because of having to invite every user on every tenant or create some kind of sync invite and removal per tenant.
Within SharePoint and Office 365 groups you need to enable external guests to give them access.
Microsoft is working on the ability to migrate companies and have some sort of coexistence. I saw this in a presentation on Ignite last year. That presentation was for Exchange.
Depending the size of the whole organization, the child organizations and the IT management.
My advise is to understand your needs, the options and when you have the final solution based on the needs that the best advice is use just one Tenant The holding company, finally have a sponsorship of the top management.
Note: I have seen in the past years the separation approach and after a few years the have consolidated to only one Tenant, because many factors (Management, Licenses, Consolidation, Security and so on)