Well, here's the wiki answer!
Note: you can't create any subgroups until the primary group gets approved. For more info on subgroups, see Why use a Subgroup for more information about subgroups.?
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4 (b) Can I transfer more than one Yahoo Group into one Groups.io group?
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Yes. The Premium Yearly plan allows multiple Yahoo Groups to be transferred to your Primary group and/or its subgroups. Be sure to transfer one at a time to avoid problems.
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If you transfer more than one Yahoo Group into the same Primary or Subgroup the additional content appends to what's already been transferred. Any members already present in the Groups.io group will not be transferred again by later transfers.
Frances
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FAQ on Moving from YahooGroupsGMF Wiki: (unofficial) Help for members (and would-be members) and group managers