It is very common in science to have multiple authors on a publication. This stems from the collaborative nature of science and is not something to be discouraged. However, an author of a scientific publication must have made a scientific contribution to the study and results being presented. Simply being a part of the same lab, or an advisor to a student or on a student's committee, paying the “bill” or providing lab space/equipment/materials, proofreading, or, even worse, in exchange for a favor such as authorship on another publication, or (the worse case I have heard of) blocking publication by a reviewer until the reviewer is added as an author and the manuscript resubmitted, are not valid reasons to warrant authorship and could be considered professional misconduct.
However, there is a fuzzy boundary.
If someone made a significant contribution that did not raise to the level of authorship they must be acknowledged. Either within the main text or in the acknowledgements section. Also, if preexisting data or materials are used cite the original publication that described them. (Pay attention to “how to cite” links on software and resource websites.) Clarity and communication here are key. Have a frank discussion with all of the authors about authorship; the earlier the better. Also, all of the authors must reasonably agree on the wording and presentation of the manuscript. If anyone is not comfortable with the manuscript they have the ability to (must be given the opportunity to) withdraw themselves as coauthors.
(Add here later a discussion of the order of authorship. The PI must negotiate between aggressive / squeaky wheel / preemptive self promotion of coauthors versus quiet humble individuals that do not strongly self advocate but may have made a primary contribution.)