There are 2 methods you can use for BGs.

1. Try and be the last 2-5 to join, by waiting til the very last second before pressing the join macro.
2. Try and be the first 2-5 to join, by mashing the macro and having a fast internet/computer as soon as the bg pops. IF any of your characters become leader, you can move any player from your server (including other randoms) but only from your server. So if you are leader, split up and in anything but IoC and AV, you can move everyone to an empty group.

If you fail at either, you can try asking the BG leader for leader or assist. Ask nicely and provide which group you are in. With assist you can also move anyone from your server to an empty group.

For WG if you dont get leader, just ask for it or assist. Because everyone is from your server, you can move everyone around once you get it so you can stay in the raid. If you end up in different raids you can kick people from your current raid and invite yoru alts with leader/assist.