MMP & Uniformed Service

Anyone here have any experience with joining the MMP or AMO while in a uniformed service?

I’ve just recently got my 1600 Mate license and would like to start keeping my eye open for work. Some companies I’m interested in, Maersk for one, require union membership to see and apply for jobs. Is it possible to join the union so I can see this work without impacting my uniformed career. I’m happy where I am for now but I don’t want to sit around with my eyes closed while good opportunities to got to sea full-time slip past.

Also curious to hear peoples opinions of whether I should or should not join a union in the first place.