If you mean how can you make people follow you, you can't. You can only encourage them by interacting and providing interesting or useful tweets.
You can search for people in your niche. follow them. tweets interesting and useful sources and messages. Hope and encourage them to follow you back.
I think he wants to ask that how he can increase followers of someone else. He is interested in getting followers for someone else's account.
If that is the case it will depend whether he has access to their Twitter login or if he has any influence over what they tweet. If he has access to login he can be useful with tweets as already said. If he has influence over what they say, then he can do the same, and make sure that they publicise the fact that they have a Twitter on their main site homepage and Facebook page etc. You can buy followers but I don't see how that is worth it because the followers you buy aren't really interested, they just boost numbers. He can recommend on his own Twitter stream for other to follow the target Twitter account. e.g. using #FF (Follow Friday). Need a bit more info for a better answer.
Following people in the same niche, like searching for digitalpoint or v7n if you are into webmaster world, and click all the followers if you have an IT profile, they will surly follow back most of them
if you wanna increase followers for someone else just start talking about them on twitter an how good they are an say good stuff just put there twitter name.
if you are aiming to get more followers. Be active and tweet interesting topic. You may also join social media exchange to grow your followers easily.
you must have an access in that account that you want others to be followed. and then post updates that makes sense. if people like your ideas/ updates, they might follow you.
twitt more interest things, photos etc and actively participate with others follow others (same niche will be help) and try to comment to any other websites/blogs using your twitter account user name.