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how would one go about taking over part of the clan?
For this to happen; you’ll need to be approved access to add/remove members from the guild. (This is an action the guild owner can do) To add onto this; the guild admin does not expire meaning they will exist into their position with no time limit.
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I know that in WoW, if the leader/guild master has been offline for 90+ days or something, you're able to petition and request that position. Does something like this exist here?
Unfortunately this is not an option. The guild owner will remain the guild owner with no time limit or able to “petition” out of its seat.
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want to invite a few friends that have recently joined JSP to the clan and cannot with my current permissions.
The guild owner will need to grant you access to this feature.
Here’s a list of FAQs:
https://forums.d2jsp.org/info.php?p=5