There are a couple different ways you can add members to your Team.  


Adding members during Team Creation

If you're still in the process of creating your Team for the first time, you're presented with the opportunity to add members during the process of Teams creation.  This works for both public and private teams.

After choosing the type of team and giving it a name and privacy designation, You're presented with this screen which will allow  you to add members by typing their name into the search bar, then clicking on the option presented below to select the user.  They will then appear in the bar in a bubble as selected member.  You can add multiple at one time this way and add them all at once. To finish adding members and launch your team, click the Next button at the bottom.  For the process of creating a team, click here view the article Get started with Microsoft Teams.

Adding members from the "Your Teams" page

If you've already got a Team established or are a member of multiple teams, you can add members directly to a specified team by clicking on the Team Icon on your left side nav bar.  This should bring you to the Teams page that will show you all the different teams you are part of.  In the top right corner of each each are the three dots "..."  

Click on the three dots "..." section to open the drop down menu, and click on the "Add Member" option.  This will bring up the same "Add Members To" box as shown above.  

Adding members from within your Team

From the image above, If you click on the Box for your team, you will enter the Home page for your Team which is typically will be the "Posts" wall, where notifications and changes are posted, we well as any mention you'd like to make.  

From this page, You should see a new column to the right of your navigation column that has your Team icon, name, and your channels.  If you don't see the bar and only see the Posts page, your window size may be small and the column is auto hidden until you either expand the window, or hover your mouse onto the navigation panel.  See Microsoft Teams - Creating a new Team for more information on window sizing and column views.

Click on the three dots menu "..."  to open the familiar drop down, and click on "Add Members" and proceed as shown in to add members to the group.

Also Note, THis can be done in the Channels section as well.  Your General channel is a default channel for the team, when adding members from the "..." above, they will automatically have access to General. If you have public channels in your team, the add members option will NOT be displayed on the "..." menu.  I you have a Private team though, as pictured above, You will have the add member option on the "..." menu.  For more information about Channels and how to create them, Please see Microsoft Teams - Creating Channels in your Team