As a owner of a Team or Channel, you are required to control who is allowed access. If you would wish to only add a member, you may refer to here.

Edit/Remove member

1. Select  next to the Team/Channel name.
2. Select Manage Team or Manage Channel.
3. Expand the member and guest list by clicking the arrow button beside Members and guests.
   
4. To change a member to be an owner, select the Arrow next to the word Member, and select Owner.
5. Vice versa to change a Owner to a Member.
6. To remove a member, click on the cross icon and the member will be removed.
   (Do note that only member can be removed. To remove an Owner, please change the permissions to member before removing.)