Usually journal will send Call for Papers to a mailing list. In my understanding, they seldom invite a particular author to submit a paper. Sometimes, PC Chair of a conference will invite authors of some selected papers to submit extended version of a particular paper to a special issue of a journal. However, it is unlikely that the editor/guest editor will invite author of poster to submit a journal paper.
There are good chances that they might be some predatory journals, which may end up asking you to pay the fee etc. One has to be very careful in sending his/her work. We can not let out brain child to be spoiled by such journals. Even though if the journal is not a spam but however publishes very ordinary level work is threat to your future, as the work will remain against your name till the end of times.
The best advice is to look the publisher, its volume number, its affiliations and then the editorial board. Renowned publishers mostly have reliable journals, the volume number suggests us the number of years the journal is in the market and the affiliations (associations e.g listed at Thomas Reuters etc. ) validated their credibility.
Best of luck to your journey of research publication ...