I recently submitted a journal article and it is under review. I have not heard back yet. I am in contact with a potential collaborator, and I think it would be beneficial for her to read I have in the submitted paper. Is it advisable to send out preprints that have yet to be published? Rejection of a submitted manuscript is always a possibility.
I frequently privately share pre-prints that have not been published anywhere (not even pre-print servers) with colleagues who I know and trust. I often won't say where it's under review, however, just because it's kind of pointless to say a place it might be rejected from.
In fields where pre-print archives like arXiv.org are used, once you've put it up in such a pre-print server, it's got a public date stamp and it's entirely reasonable and safe to share with anybody. Likewise, once a paper has been accepted and is just waiting to appear, I feel it safe to share pre-prints privately.
With something that is under review and not on a pre-print server, however, I would not share it with people who I do not already know and trust.