Login to Microsoft teams and click on teams on the left.
Select the Teams main Channel in which you want to add member.
Click on the 3 dots
Click on the option add member
When you click on add member, new window will pop-up as below, enter the email of the user you want to add.
Once you type the email ID click on Add
To add member in the Sub Channel
Select the Sub Channel and click on the 3 dots more options
Click on add members as shown in the below image
Type the email address and click on add.
If you want to add more member to the Sub Channel.
Type the email address and Click on add.
To add the users in the groups.