I've always treated memberships as
unallowable on grants, as their benefit is not applicable 100% to that grant and not needed to complete the research goals. In addition, I mostly have federal awards and their policy typically communicates these are indirect costs. However, as discussed in this back and forth with Nicole, I
do allow memberships on grants and consider them the same as conference registration fee
IF member registration fee + membership fee < non-member registration fee. That is the only time I've allowed it on grants, requiring backup from the conference website showing the breakdown of all these costs.
For example, some conferences will be $800 for non-member and $200 for member (and membership is $300). Therefore both the conference fee and membership fee are approved and coded as Sponsored Conference Fees. Not sure if this helps your exact situation, so feel free to reach out further!