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