Setting up a Multisite instance on DreamHost isn't as easy as it should be if you want to also have SSL certificates, but it's possible with the right settings.
TL;DR: Loop through all sites and delete_option( 'rewrite_rules' ). Don’t call flush_rewrite_rules(). Full example. Every once in awhile I’ll run into a situation where something will break permalinks on all the sites in a WordPress Multisite network, like a plugin network-activation gone wrong. On a single site, it’s easy enough to fix by manually visiting Settings > […]
Warning: This hasn’t been updated since 2011. It should still be somewhat useful, but you’ll likely need to do additional research and testing on changes since then. Migrating multiple standalone installations of WordPress into Multisite can be relatively easy if you don’t need it to be perfect. The Codex has a basic guide and Stephanie […]