I read this series of novels in paperback more than 40 years ago. There were at least three books in this series, but I'm not sure exactly how many. Not more than six, I believe.
All I remember is that a team of space explorers, men and women, would visit various planets to open them for colonisation. But if they found any intelligent life on a planet they could not claim it for mankind. It was not always quite clear, however, as to what constituted intelligent life. Interactions with whatever life existed on these planets were usually complicated. Alas I don't remember much else.