@TheMask
If users create a community in Yammer, and they have rights to create Office 365 groups, it will create one. This is referred to as a "connected group", if you have disabled the ability for end users to create O365 groups it will create an "unconnected group" which only exists in Yammer. A good document that covers the different caveats is here
Currently you can not create Yammer communities from existing O365 groups they must be created inside Yammer. Also an Office 365 group can't be linked to both a Team and Yammer at the same time so you need to have O365 groups for the Team and for Yammer.
One way to link Yammer in Teams in a useful way is say you set up a Community in Yammer called "Yammer Support", that has the purpose where people can ask questions and help each other user Yammer. You might also have a group of people involved in the project for rolling our Yammer and you might have a "Yammer Rollout" team in MS Teams. You could use the Yammer connector to a post a message to the in a teams channel whenever a user creates a post in the community, that way it will get the attention of the Yammer Rollout team quickly and they can discuss how to respond inside the team, and then post a response in Yammer
