I understand how to invite the administrator of a tree you have a SM with to be a member of your tree, and I know how to request membership on another tree without a SM as well, but I can't figure out how to invite another myheritage member who does not have a match with your tree.
To invite another MyHeritage.com member you should know the email of his/her account. Then, enter to the section: 'Home' > 'Invite more family and click on the link '-Invite more people manually', located on the lower part of this section.
Hope this clarifies. Please contact me if you have further comments or questions.