I can say from my perspective there are plenty of scenarios besides calling like you already described. Invest some time and talk to the people you want to put in that user group for testing and listen to their needs. (in my opinion, this is most important regarding adoption and "user happiness")
Often a good structured Team with people along the process (not functions) helps people a lot and saves a ton of time for everyone in that Team!
Introduce guys who are often on the go into the mobile app and take time to set things up together. It is a good idea to take for example sales people from inside and outside of the office (which are working a lot together) and show them how to enhance communication.
Hope this helps you! There is a ton of scenarios but please do listen carefully to what the people you are "serving" want to do!
maybe you already know this page, in case you don´t take a look!
@Deleted If you think you will ever be inviting 'guests' into your tenant - consultants, vendors, etc, have one of your test people become a 'guest' using another email address - have them create a @outlook and/or a @gmail account. There are some things that work differently for guests that might catch you: