West Midlands-based Top Banana has hired Slevin as client engagement director and Silvester as venue co-ordinator to work with clients including Wilkinson and M&S.
Slevin previously spent four years working as customer relationship manager at behavioural change business Sewells.
Before that she was head of sales operations and planning at O2 for almost nine years.
Top Banana managing director Nick Terry said: "Maggie has spent the last four years helping directors and managers of national and global organisations to amplify, accelerate and execute their strategic plans for growth and cultural change. Her skills will be an excellent addition to our existing team."
Slevin started her role this week alongside Silvester and the new appointments increase Top Banana’s staff to 22.
Silvester previously worked as an events manager at Ellis Salsby in Kidderminster, Worcestershire, where she spent 17 years.