Whether or not an Organizer or group is required to pay taxes on membership dues or event fees collected depends on many things, including the status of the Organizer and what other activities the Organizer is engaged in.
Generally speaking, individuals must pay income tax on personal income and businesses must pay income tax on business income. But that does not necessarily mean that an Organizer has to pay taxes on all money received.
For an Organizer that is an individual taxpayer, dues or fees are taxable as income. This is true even if a Meetup group is a "hobby." Federal tax law does distinguish between an "activity engaged in for profit" and a "hobby" but income from both are taxable.
However, federal law allows a taxpayer to deduct expenses in connection with a hobby (but only in an amount up to the income derived from that hobby). So, for example, if an Organizer collects dues or fees simply to pay for group-related expenses, that Organizer will not have to pay taxes on those dues or fees if the group expenses are at least equal to the total amount of dues or fees collected. However, an Organizer may be required to report the income and expenses on their tax return even if the income and expenses cancel each other out.
For an Organizer that is an entity, dues or fees may be taxable as income, depending on the type of entity. An organization that is a non-profit corporation is likely a tax-exempt organization and thus is not required to pay taxes on its income. Note that even if an organization is a non-profit incorporation, a separate process with the IRS is still required to qualify as a tax-exempt organization under the federal tax law.
For an organization that is a for-profit entity, such as a corporation or LLC, dues or fees may be taxable as business income. Of course, this income may be offset be appropriate deductions for business expenses.
Regardless of type of organization or income and even if the income from dues and fees is only to pay for group expenses, an Organizer may be required to report the income and expenses on a tax return.
Note: This is general information and not legal advice about any specific situation. Also, in addition to liability under federal laws, each state has its own tax laws that may be different from federal law. You should consult an accountant or a lawyer for advice about your tax liability and your specific situation. As an additional resource, you might consider checking out 1099.is. 1099.is provides general answers to people with freelance tax questions. You can either ask a question here or review answers to past questions here. Keep in mind that 1099.is does not provide specific tax advice, and you should speak with an accountant before making any final decisions.