To Add member in the Microsoft Teams and channels

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.