Nairobi is 1 hour ahead of Brussels
Best overlap: 8:00 AM to 5:00 PM in Brussels is 9:00 AM to 6:00 PM in Nairobi.
Book within 8:00 AM to 5:00 PM Brussels time (9:00 AM to 6:00 PM in Nairobi) and both teams will be at work. That's a roomy 9 hours of common time each day, so finding a slot that suits everyone is rarely a problem. Daylight saving complicates things slightly: Brussels observes it, Nairobi doesn't, so the difference flexes by an hour part of the year.