While there is a "Reimbursements" type on the income tab, you can also track reimbursements a different way to offset your original purchase. Here's how:
Say you and a friend to out to dinner and you pay the bill. You log that in the Use of Money as Wants > Takeout/Dining Out. Let's say for this example it's $100.
When your friend reimburses you, track the reimbursement as a negative amount in the Use of Money. So if they pay you back $50 for their half, you would enter the amount of $50.00 (make sure to add the minus () sign!), then categorize it as Wants > Takeout/Dining Out. This way, when you look at the summary of how much YOU spent, it will only show $50.00.