Visit to login to your account, from your dashboard go to repositories and select 'Packages'. Upload the android .apk app. A package can have any number of apps in it.

Select devices on the left menu and double click the devices you wish to deploy the app to. Select the red manage button and then 'Tags'. Click where it says 'Add Tag' and write your desired tag name, press enter to confirm.  You can then close this menu.

Open 'groups' from the left side menu. Select the group, click on the 3 dots which opens the options menu. 
Click 'Install Package', select the package and click apply.