If UNA site has the installed Spaces and the member has already own spaces or spaces where he already joined, then every "Add new" form (album, photo, group etc) in the "Visibility" (or "Privacy") field will have the "Spaces" variant sub-header with the list of available spaces.

The best way to use features of the Spaces app - post a content via the "Create Post (Context)" form. Just add it via Studio->Pages->Add new block area on any page and touch it :-) Like we have in our https://online.me homepage.