This message essentially means that the server is full and is not accepting any more clients.
The best thing to do might be to simply try another server on the same network. If you want, you can also keep trying to connect to this server (under the assumption that there are people who will be disconnecting from the server all the time) but this may not prove to be very effective.
First of all, some networks (such as DALnet), will automatically ban you if you try to connect too many times within a certain time period.
Secondly, some servers may be restricted on purpose. In an effort to lower user count, the server administrator may have decided to restrict the allowed number of clients to a number that's a lot lower than the actual number of people currently on the server (the idea is to lower the number of people on the server gradually, without kicking anyone off). While rarer, this is still a possibility.
However, there aren't any tricks that you can use to get on a server that is full (unless you know the server admin, but that's another discussion). All you can do is to keep trying the server and the other servers on the same network.