Then your "passenger" roster is full... BUT... no harm done... Save the assignment you have untill you've started the colonial chains.
Thers one in each explore sector, and you can recognize them by the <mission objective (x/7)> under colonial.
once you finish theese chains you will get the chance to earn good quality DuOff's, but you need alot of Colonist to complete the chains, so stick with them.
Of cause you can always sell them for dilithium (the ones you have in your passenger list).
You can also use your in-game mail as Colonist storage - compose a mail to yourself and attach 5 Colonists to it, and voila! I do this all the time, although I use it more to transfer them between my characters.