But, if that person has a team of supporters behind them, whom they regularly update, they are more likely to stick with their new habit during times when they are establishing their new habit and motivation is running low.

【答】本句用behind him 或behind them，都对，只是意思不同。正如网友所言，behind him 是支持that person 的，但behind them 则是另指他人，是复数“他们”，即包含that person 在内的一个群体。此外，从后面的定语从句whom they regularly update 即可发现，里面的主语是they，而不是he。