Go to the Settings module by clicking on the icon from the drop down menu on the top right or from the dashboard
Then click on the Groups icon in settings to open a page that should look something like this:
Find the name of the group you want to add a person to and then click on the button with the person icon 
.
Another window will open with a list of those currently in your group. On the left hand side you will see a search box. Type the name of the person you want to add to the group in this box. You will see their profile appear in the space below. Click on this and they will be added to the list of members.
You can then search for the next person you want to add.
You can also assign each person in the group permissions here. These are usually only for group leaders but you can also give someone else in the group the ability to Take Attendance or Send Messages if you want to.