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.

• Does providing samples you have previously collected for another study raise to the level of co-authorship? In general it does not, there is an understanding in science that materials are made available to other labs for additional studies, but de facto this is sometimes used as a reason for authorship. Collecting samples / generating data for the study, however, is a critical part of scientific research and a valid reason for co-authorship.
• Does providing data warrant authorship? No, the culture of science is to make (non-protected) data freely available to anyone who requests it (however, there are reasonable limitations such as personally identifiable information of human subjects protected by confidentiality). Providing materials and data is also central to replication of an experiment and independent verification of results in addition to enabling meta-analyses of multiple publications.
• Does coming up with the idea for a project raise to the level of authorship? Yes and no, this is really up to the authors that carried out the study. Frankly, ideas are cheap and one can't simply stake a claim in science by proposing an idea for a project. An important part of science is to suggest studies for other people to carry out without strings attached. On the other hand, many projects would never be done without these enabling suggestions to frame the question.
• Does proofreading, assisting in writing the manuscript, or assisting in designing a figure, count as a co-authorship-level contribution? No, this is not a scientific contribution.

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.)