Navigate into the Group you wish to add to. Tap the ‘more’ menu (three verticle dots in the top right of the screen). Tap the “Group Info” option. Now in the Group Info screen, tap the “+” floating button in the bottom right of the screen. You’ll see a listing there of all members in the community with a grey check next to those already in the Group. Now simply tap on the avatars of those members you wish to add now, and when you’ve finished adding new members, tap the “right arrow” button at the bottom right to finish the adding process.