- Go to the MANAGE > Groups.

- Open the group you wish to change.
- If you have not set the admissibility of being a member of multiple subgroups yet:
- Click on settings to the right of heading Administration.

- Click on edit in the section Members rights.

- Check the square box in front of Users can be members of several subgroups.
- Confirm by clicking save.

- Click +ADD after entering the group page and select Invite user.
- Even if the member is already part of the group, type in the member’s email address and select which subgroup you'd like to invite them to from the list.
- If the member is not yet part of the group, type in the user’s email address and choose the respective subgroup you'd like to invite them to from the list.
- Confirm by clicking INVITE.

- Repeat step 4 and 5 to add the member into more subgroups.
Note: Find more information on Group management in this article, please.