Greetings from CA, first time I've gotten online since Friday.
The designation of defense or offense is going to depend somewhat on what class you choose, but more so on how you play that class. It's hard to peg people into playing the class as one role without restricting the player to zones, ala soccer or even baseball.
It might be interesting though to have an automatic icon appear next to a players name if they are close to the base, you could hit tab, see that there are 3 players with a hive icon next to their name and quickly conclude there are enough people near base. Or vice versa, you could see no one there and suggest something be done.
The booster is one way of encouraging players to defend though. It's a supportive structure that gives a temporary damage bonus to any alien touching it. It's timed, so it's only useful near the base. Basically it makes any aliens near the base stronger , if they use it. So humans should assume it's being used and take caution. It does not last long enough to use and flee to the human base. Although you could make a booster outpost, it would set itself up to be attacked easier by the humans.