Is it possible to manage an excel spreadsheet online and have additional people have access to it?
Use Google Docs
I will second Wayne's recommendation. Google spreadsheets are simple to access and use. Let's face it everyone has a gmail account (unless they're stuck at AOL) and what you typically want with a shared doc (spreadsheet or otherwise) is easy access.
The one caveat is around complexity and high order functions. If the spreadsheet is highly complex with rarely used excel functions, it may be easier to go with office 365 per Jaime below. Google spreadsheets are great for about 98% of excel functionality. However, when you need that final 2%, it pulls up short. That's exceedingly rare, however.
Or Office 365 - which is
The latest features of Office 365 include real-time multi-user editing. If two people are editing the same Excel spreadsheet at the same time, the cell that the other party is editing shows up on your screen with a thin red outline.
If you want collaboration that doesn't necessitate such up-to-the-second information, try Dropbox or Box with a desktop sync. Two people can both access the file and work in it as long as it's not at the same time.
Google Docs is workable as long as some of the niftier features of Excel aren't needed.
Take a look at Excel Web App. Here's a description for Excel 2010:
And here's a blog that describes the Excel 2013 version:
Also, another possibility is using shared workbooks, as described here:
Honestly, I haven't tried any of these options. But they seem to be popular solutions.