If I were you, I would get a membership and list your own city and maybe 1-2 fringe cities/towns (where the big boys don't care to appraise because they like the convenient local work). Chances are if you list yourself as covering one of the larger cities, you won't stand a chance in competing for assignments against the experienced ERC guys - so that money spent would be wasted. As others have said, time and experience is the main way to develop repeat work in this field. Your initial goal should be to get your foot in the door and some ERC experience under your belt.
Study your ERC guide long and hard on forecasting and how to adjust for depreciating values. Unlike in the recent past, most of the country is currently experiencing high home inventories and declining values in many instances. You have to understand how to adjust for this stuff or all of your time spent getting into the field will be wasted, because inaccurate values will bring a quick end to your relocation appraisal career. Strive for excellence because if you make a mistake (missed concession, square footage error, insufficient forecasting, etc.) chances are the other appraiser won't, and you will hear about it when it comes time for the review. You don't want to be the one who appears to be the amateur. Most relocation companies also now use actual appraisers (some still don't) to review each assignment, and they are pretty sharp about catching common appraiser mistakes.