Anyone in a group can add another from their contacts to that same group, not just the admin of the group.
Note: you must have already established contact with a Session ID before you can invite them to a group.
Mobile:
- open the group
- click the settings button (3 dots)
- click 'edit group'
- click 'add members'
- select the contact you wish to invite
- confirm your selection
Desktop:
- open the group
- click on the group's profile picture
- click on the 'human' and '+' icon
- select the member you wish to invite and confirm with 'ok'