First, have a look over this article:http://office.microsoft.com/en-us/access-help/ways-to-share-an-access-database-HA010279159.aspx#BM2
Sharepoint is probably your best bet. There's a ton of ways to share an access database online, some are costly, some are complicated.
Readyshare, will not work. It will still be hosted on one facility causing uploads and downloads constantly to keep the data fresh, but it will be locked by the host until completed (you, for example), simulatneous access will not be possible, and it would be slow even if it is.
You need a server host to support the bandwidth of people connecting.
Sharepoint however is not overly complicated. This is a breakdown of the setup (using 2010):http://office.microsoft.com/en-us/sharepoint-server-help/build-an-access-database-to-share-on-the-web-HA010356866.aspx
Honestly, it's going to be your least painful choice but you'll need a host (many are available online), and it will cost money. Any feasible option will.
You could of course buy a sharepoint server and host it at your place, but it'll be just as costly and not as reliable.
Here's an example of one host:http://www.rackspace.com/enterprise_hosting/sharepoint/multitenant/
Its priced at data used, not at user level (seat's generally go for 150$'s a piece, so at their current pricing at rackspace, that's cheap at 10$ per month per gb or so of data).
Anyway, a lot of reading to do on it. Contacting one of the hosts to see if they can meet your needs and at how much is your next step. Most of the hosting providers will walk you through the process as well, very easily.
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