You can use MS Teams to Chat with one person or more than one person in small groups. These are great for sub-working groups and class project groups.
Note: Group calls via Chat are private and will not show up in Channel history. These are not part of a Team. So if you have a file that is saved in a Microsoft Group, that you want to have this small number of people work on, you'll need to either set up a private Channel in your Team or just link to the file in the Chat.