To put a password on your groups section on your home tab as shown below.
Once in the groups section click on one of your groups that you wish to put the password on and click on the avatar.
You will then be directed to your groups homepage, Click on the 3 dots at the top right of your screen then click edit when the option pops up on your screen as shown below.
You will then be directed to the groups editors page click on password optional and then type the password you wish to use. Then click save and accept the Palringo guidelines as shown below.