How can I motivate a growing sales team?

This is an enormous question - there have been entire books written on how to motivate a sales team, and for good reason: if you can setup the internal dynamics of the team correctly, you can double or triple the sales you'd see in a group of "average" performers.

Here are some techniques used in company's I've worked at:

1. Determine Achievable Goals. You probably already have a couple of sales reps, so do an average of their daily/monthly/quarterly sales to set some benchmarks for new staff. Take into consideration training & ramp-up time so that they have achievable goals.
2. Commission + Benchmarked Bonus Structure. Your average salesperson makes $X for on-target sales. If they sell more than that, you might consider increasing their per-sales commission (either for the extras or all their monthly sales that pay period). That gives them an incentive to push harder than simply meeting their goals.
3. Benchmarked Penalty Structure: If you have bottom performers that you want to try and salvage, consider reducing their per-sale commission if they don’t meet a specific sales quota.
4. Replacing Low Performers: If you have a few folks who just don’t have it together, consider replacing them altogether. First, it drags your whole team down having a couple of lame ducks dragging along the bottom, and second, it sends a message that lack of performance is not acceptable.
5. Gamify your sales process. The simplest tool is simply a “Top Gun” leaderboard that lets them see who’s ahead, who’s behind, and how they compare for each pay period. Consider some kind of bonus for each month’s #1 salesperson.

You should carefully examine the specifics of each point I've mentioned to see if (and how) it fits in your team before trying to implement it. That's especially true when it comes to bonuses and penalties - there may be legal or franchise regulations involved with this in a real estate brokerage that don't exist in other industries.

Similarly, replacing low performers is handled in different ways. In real estate, some brokerages charge agents a "seat charge" for office space, and if the agent doesn't perform, they remove themselves from the team to save money on that monthly fee.

On that note, one thing worth avoiding is "cat and mouse" games with your sales team. Motivation is one thing; outright manipulation is another. Outline goals, stick to your word, and set truly achievable goals - or else your sales team will find a more lucrative company to work for.

Remember, sales people tend to be more independent than other professions - especially in real estate, where most of the time they're considered independent contractors. On the one hand, that makes them easier to replace, but on the other hand it means they're not likely to stick around unless you're creating an environment for them to succeed in!