When sending individual invites, the invitation link and code are person-specific. Each recipient of this link or code will establish a connection with you only, and their results will be shared only with you as the person who sent the invite.
On the other hand, group invites are group-specific, meaning that the link or code is intended for a particular group. Joining a group using this link or code shares a person's results not only with the creator of the group but also with the other members within that group.
If you wish to add a person to the group, make sure to send them the invitation to join the group. The individual invitation will only connect them with you, not with the group.