Yes, they can; but in this forum they can post in fellowship, not as full members (as is true of all of us who are not Messianic); and they must not debate or teach against the Statement of Faith.
There are Rabbinical Jews that have a preferential status here in MJ. We love them to death. But they do not post as full members-- they can only post in fellowship, despite the fact that there are those of us who wish this were not the case. Non-Trinitarians know they cannot promote their non-Trinitarian beliefs in the MJ forum. There are specific forums that are designed for non-Christians where non-Trinitarian dogma can be discussed freely. This has been the case for a long time, despite the fact that there are some in the forum who wish this were not the case.