SLM's aim is for Everyone Active members to do 30 minutes of activity five times a week, two of which should be in an Everyday Active centre.
Black Sun has been appointed to review SLM's data, proposition, member benefits and communications.
The agency will then work with SLM to create a CRM programme that maximises member value and achieves its business objectives.
SLM offers a wide range of sport, exercise and activity programmes to help members lead a more active and healthy lifestyle.
Neil King, commercial director at Sports and Leisure Management, said: "We have a unique proposition in the market and look forward to using Black Sun's expertise to communicate this more effectively."
Richard Dixon, director at Black Sun, said: "We're delighted to be working with SLM to enhance their member management and grow member value."
Black Sun has worked with some of the UK's leading brands including British Airways, Barclays, Eurostar and O2.