United operates its own United Clubs worldwide; for a detailed list, see this FT thread: United Club and Other United Lounges.
Official lounge access policy on UA's website: Club access policy
Older archived FT thread: Consolidated "United Club Access Questions" Thread [ARCHIVE]
For UA partner and Star Alliance lounges, reference the *A website: *A Lounge Finder and *A Lounge Access Policies. Note that some airports have "contract" lounges with varying access rules. Also see Non-UA Lounge Access Rules for United Club members?
United also operates Polaris Lounges and Arrival Lounges and previously operated International Global First Lounges (respective FT threads linked).
GS2021 newsresent GS status extended to Jan2022, domestic UC access, additional PP

To have access to the UCs, you will need to fit into one of the following:

• UC member with same-day UA, *A, or contracted partner BP* UC members can have 2 guests or guest their spouse and dependent children under 21 years (note at non-UA *A lounges, UC members are allowed only 1 guest)
  * Since 1 Nov 2019, a same-day BP on a UA, *A, or contracted partner flight is required for entry
  * Actual UC membership card is generally not required at UCs if presenting a UA BP with member's UA FF# as the UC membership is tied to the MP account
• UA *G member with international *A BP, for any class of travel, showing status
  * Can guest 1 traveler with a departing *A BP.
  * UA *G cannot access domestic UCs unless traveling on a *A international itin (within 24 hours - some agents use same calendar day) - can be different PNR
• non-UA *G member with same-day *A BP for a flight departing from the same airport, showing status
  * Can guest 1 traveler with a departing *A BP from same airport.
  * If non-UA *G but crediting to UA MP, you will need to present your non-UA *G card. There have been some reports of refused entry -- agents claiming you have to be crediting to the same program as you are requesting entry
• passenger with same-day *A BP for international travel in business or first class, departing same airport
  * International First passengers can guest 1 traveler with a departing *A BP
  * No guests for International Business.
• Premium transcontinental non-stop (EWR-SFO/LAX & BOS-SFO) business class traveler
  * Limited to origin and destination airports of the premium transcontinental flight -- does not include connecting cities
  * No guests
• Have GS status with a same day BP (UA/partner carrier)
  • No guests
• Have or purchase a UC one-time access pass with same day BP
  * No guests
• Have Amtrak Select Plus or Select Executive status.
  * 1 guest and/or dependent childern under 21
  * Same day BP on ANY airline (arriving or departing)
• Active U.S. military personnelActive duty military members with a valid military ID, boarding pass for travel within 24 hours on a United- or United Express-operated flight and one of the following:
  Dressed in uniform
  Leave orders
  Rest and recuperation papers
  Access is subject to United Club seating availability.
  Allowed Guest -- Family members with gate passes or traveling on the same flight as the military member
• Air Canada Maple Leaf™ Club membership with a departing *A BP from that airportMaple Leaf Club North America members can access United Club locations in the U.S. only. no Guest.
  Maple Leaf Club Worldwide members can access all United Club locations. 1 guest.

When BP/ticket is required, it can be either a paid or award ticket.


United Club T&Cs

Common questions
Children -- Are they counted as a guest?
Anyone with an individual ticket is counted as a guest. Lap children (below 2 years of age) are not counted as a guest. UC members can guest their dependent family at UCs (but this will not work at other *A lounges).

Is Canada, Mexico, Caribbean, Puerto Rico (San Juan), .... international for UC access?
Yes
What about Domestic First Class?
Domestic First Class is not provided with UC access. Unless you qualify based on some other reason, there is no access for paid or award domestic FC.
Why no United Lounge access with UA domestic first class tickets?

Can my guest stay if I leave the lounge?
Strictly speaking, no. Guests only have access when the sponsor is in the lounge. You have "responsibility" for the guest when they are in the lounge. Realistically, however, there is usually no problem with leaving a guest behind.

Can access to the UC be further restricted?
UA reserves the right to limit access including the number of guests at times of crowding.

Can I bring food into the UC?
Strictly speaking no. There may be local health & safety codes against this.

Can I guest 3 co-workers by using multiple access methods? Such as 2 via UC membership and 1 as *G international?
No -- you cannot stack access methods.

Can I access the UC on arrival and no forward *A travel or if departing on a non-*A carrier?
Maybe if UC member or UA *G on international itinerary or arriving on UC operated Polaris cabin itinerary (Also you will need to able to access to the UC location)
As of 1 Nov 2019 - all will be required to will need to provide a same-day boarding pass for travel on United, Star Alliance™ or a contracted partner for entry into all United Club location. Other airline BPs will not qualify
Club member/one-time pass access changes Nov 1, 2019 (same day BP on UA or partner)

Partner lounges
USA Lounges with UA *G / UC membership Access When Flying Domestic
Access / reviews for TK's IAD *A lounge
Consolidated "Access to Lufthansa Lounge at IAD" Thread [2014 forward]
Best IAD Lounge
Lufthansa Lounge Access in DTW/Detroit
Accessing the *A Lounge at LAX/TBIT as a UA flyer

Consolidated "Polaris Lounge Access Questions" Thread
Non-UA Lounge Access Rules for United Club members?
The United Club Meet-Up Thread