If the group you're in is discoverable (i.e. can be found via search or the directory) you can invite in members.
- Go to the group home screen
- Tap 'Invite to group' in the banner at the top of the screen
- Copy the link.
In discoverable groups hosts have the final say in who can join the group based on the information in their profile. You can share the link via email, on social media or on websites so others can request to join.
If the group you're in is private you won't be able to invite new members. You could contact your host to suggest someone you think they might like to invite.