You can read their rules carefully and then apply to those campaigns but if your rank is low you can increase your rank by posting and then again be a part of that campaign.
Everyone is allowed to apply for the campaigns that are open and the ones who contribute good stuff may get accepted in a campaign.
The campaign managers know which members are active, they also look for the posts which contain good and valuable information.
There is of course that rank requirement and if we keep that in our mind then higher rank members will get somewhat high pay rates in some signature campaigns.
It's always better to make posts without caring about the rank because if someone posts only to rank up then such type of posting may let a member to do spamming.