Currently, when a region is full, access to the region is simply on a first come first served basis. This leads to a sort of distributed race to enter the full region; users repeatedly try to teleport or cross into the region and fail until someone leaves. The next attempt made by anyone succeeds, which makes the region full again. Even the region owner and estate managers must compete with other Residents to get into their own region if they find themselves outside of the region when it is full.
Linden Lab is introducing some improvements to how region limits and region entry work. Read the full article HERE