Add a "Buddy List"
Many/most of the chat clients I have used over the years have the option to add people to a personalized "buddy list" (it has different names, but pretty much the same functionality). This is NOT the same as a Team. These are people you want to have frequent conversations with, but may not work on the same things you do on a regular basis. For instance, the administrative assistant is not going to be on the Team for various projects, but we still need to get in touch with her frequently.
Someone mentioned to me that you might be able to do something like this through the web interface, but - even if this is the case - it will not work for us. The functionality needs to be in app, or it might as well not be there as our user base would never use it. This along with Mac parity have been the only things keeping us from adopting MS Teams (which is what my team REALLY wants).
Brian Higgins commented
I believe this already exists in the Teams app. When in Teams chat, click on "contacts" next to "Recent", and you can add users to a contact "Buddy List"