@Remo - One consideration when it comes to team Member vs. Guest is there are a number of limitations for Guests, including:
- cannot start voting sessions
- will not have access to any installed apps that require authorization
- cannot use Smart Meetings (or any other features that required the user to be a board owner/co-owner) -- on that note, they cannot be assigned the co-owner role
- no access to the board content settings
- no access to protected lock
- cannot access the board Sharing settings
Those are just a few - a full list can be seen here:
https://help.miro.com/hc/en-us/articles/360021415119-Collaboration-with-guests#h_01FJBPP0ZC7QQQY0CM45BMQ68S
In summary, if the users are just being expected to contribute on boards, being Guests could work. Even in limited scenarios, facilitating on these boards as Guests would work as long as none of the features noted above are required.