This plugin allows you to limit the number of users who can be on your server by rank. So if you only have 20 slots and want to make sure guests never take more than 5 slots to reserve the rest for your regular players, you can do that.
In order to take advantage of the group login limits, you must have a permission framework plugin installed on your server: your choices are Vault or WorldEdit (using WorldEdit's WEPIF framework). This is because native Bukkit "Superperms" does not support the notion of offline permissions. Both Vault and WEPIF support offline permissions and both support just about every permission plugin out there.
Note that some (many/all?) permission plugins (I use and tested this with PEX) will propagate permissions up to higher ranks. So for example, if you want to restrict Guests, you might think to use a "group.Guest" permission. That's fine, but be sure to negate the permission at every higher rank, otherwise every rank will inherit "group.Guest" and your limits won't work like you'd expect.