A constant recurring issue that I often encounter with many of the companies I work with is that their end users find it annoying that they often have to log in multiple times when they work in Office 365. This also applies to those who have just started using Microsoft Teams.

I want user adoption to work as smoothly as possible and I think one of the key success factors is that the end users should experience a seamlessness between Office 365 services / workloads. In order to accomplish this, it is important that you initially always begin to ensure that all end-users have the necessary Office 365 URLs and required Trusted sites rolled out on their clients.

More about this topic here:  https://techcommunity.microsoft.com/t5/Microsoft-Teams/When-multiple-logins-are-a-problem/m-p/184710#M13282