Merge the PDFs is ok. All team members can have local data. Another possiblity is that the team members prepare an InD package that holds all their data. Those can be Placed into the final InD document. The third possiblity is that one person holds the layout and others produce the content only (text files, images, tables). That needs continuous management and communication and can cause problems between the ears for those who want to have more power.
Already in other comments and answers there was a proposition of a cloud for all data. That can work, too.
BEWARE: One writes something to a file and saves it. Then another saves his own version of the same file. This is the basis of total chaos. This can be accidental, too. A password protection against this is a possible precaution. Separate workspaces in the file system prevent this naturally, but the co-operation needs also some common workspace.
There must be an agreement of some rules.
- physical page dimension settings for printing
- color space
- master pages to be used
- spread wide layouts possible or not possible?
- consistent fonts and typographical styles
- idea of the page numbering (section based numbers are possible; A1, A2, A3... and B1, B2, B3...)
- index entry principles
- available space per a member
- how to refer to other's part - is it needed at all?
- how to communicate error-freely about common things
Page numbering is possible when combining the PDFs, but that can create a mess because one can have some page numbering reference in his part.
An early discussion with the printhouse is a good assurance against a major rework and blank pages. If they do earn your money they must be patient and give some good data to the beginners.
ADDENDUM: All linked files and fonts walk along the *.indd layout file if you prepare a package (File > Package) It's a folder that holds all of it. If you want a single file, make a ZIP. I have not found how to embedd all (not only the coarse previews) into the *.indd file. The package gives a possiblity to edit something lately - you might find a too dark photo, for example. The space requirements wouldn't be smaller with all-embedded objects.
If you move the package, the links get broken. Relinking one file fixes them all. InD is that clever. Don't miss the embedding!