Please ensure you have followed the guide "Installing Teams Drop In App" first.
1) Launch the app by clicking "..." in the menu on the left and choosing "Teams Drop-In"
2) Click on "New drop-in"
3) Select the school you require by clicking the arrow to the right of the name
4) Locate the class you need (you can use the search box at the top as well) and then click on the arrow on the right hand side.
5) You will now be presented with a few different options, complete them as per your requirements. You can use the "Someone else" option to add someone who is not you to the class.
Owner - This is adding the user (you or someone else) as a teacher
Member - This is adding the user (you or someone else) as a student
5) Click on "Schedule drop-in" this will then process through and take a few minutes.
In teams you will receive a message from somebody called "Flow" telling you when you have been added to the team.