Depending on privileges defined by an administrator or group managers, the ability to invite new members to join a group is available to either all of a group's members (except guest users) or only to group managers.
Why invite members to join an open group?
Even though an open group is, by definition, open to all of an organization's members, inviting a user to become a member of a group ensures that the group will appear in a user's left-hand navigation menu, and that your group's posts will appear in their news feed. This allows you to increase the visibility of your group.
Inviting members to join a group
Click on the Users button of the group
Click on + Invite and choose to invite existing users, new members or new guests
If users are already on the platform, enter the first few letters of the name of the user you want to invite or paste a list of email addresses
If users are not already present on the platform, enter their email address and select groups they will join.