diff --git a/packages/auth/src/team/Team.ts b/packages/auth/src/team/Team.ts
index bddf0a31..d2fbbb51 100644
--- a/packages/auth/src/team/Team.ts
+++ b/packages/auth/src/team/Team.ts
@@ -555,14 +555,34 @@ export class Team extends EventEmitter<TeamEvents> {
     return invitations.validate(proof, invitation)
   }
 
+  /** Check if userId and userName are not used by any other member within the team. */
+  public validateUser = (userId: string, userName: string) => {
+    const memberWithSameUserId = this.members().find(member => member.userId === userId)
+    if (memberWithSameUserId !== undefined) {
+      return invitations.fail('userId is not unique within the team.')
+    }
+
+    const memberWithSameUserName = this.members().find(
+      member => member.userName.toLowerCase() === userName.toLowerCase()
+    )
+    if (memberWithSameUserName !== undefined) {
+      return invitations.fail('Username is not unique within the team.')
+    }
+
+    return VALID
+  }
+
   /** An existing team member calls this to admit a new member & their device to the team based on proof of invitation */
   public admitMember = (
     proof: ProofOfInvitation,
     memberKeys: Keyset | KeysetWithSecrets, // We accept KeysetWithSecrets here to simplify testing - in practice we'll only receive Keyset
     userName: string // The new member's desired user-facing name
   ) => {
-    const validation = this.validateInvitation(proof)
-    if (!validation.isValid) throw validation.error
+    const invitationValidation = this.validateInvitation(proof)
+    if (!invitationValidation.isValid) throw invitationValidation.error
+
+    const userValidation = this.validateUser(memberKeys.name, userName)
+    if (!userValidation.isValid) throw userValidation.error
 
     const { id } = proof
 
diff --git a/packages/auth/src/team/test/invitations.test.ts b/packages/auth/src/team/test/invitations.test.ts
index 36347852..e08f8d26 100644
--- a/packages/auth/src/team/test/invitations.test.ts
+++ b/packages/auth/src/team/test/invitations.test.ts
@@ -80,7 +80,7 @@ describe('Team', () => {
         expect(bobsTeam.memberIsAdmin(bob.userId)).toBe(false)
 
         // 👳🏽‍♂️ Charlie shows 👨🏻‍🦲 Bob his proof of invitation
-        bobsTeam.admitMember(proofOfInvitation, charlie.user.keys, bob.user.userName)
+        bobsTeam.admitMember(proofOfInvitation, charlie.user.keys, charlie.user.userName)
 
         // 👍👳🏽‍♂️ Charlie is now on the team
         expect(bobsTeam.has(charlie.userId)).toBe(true)
@@ -336,6 +336,58 @@ describe('Team', () => {
         ).not.toThrow()
       })
 
+      it("won't accept proof of invitation with a username that is not unique", () => {
+        const { alice, bob } = setup('alice', { user: 'bob', member: false })
+
+        // 👩🏾 Alice invites 👨🏻‍🦲 Bob by sending him a random secret key
+        const { seed } = alice.team.inviteMember()
+
+        // 👨🏻‍🦲 Bob accepts the invitation
+        const proofOfInvitation = generateProof(seed)
+
+        // 👨🏻‍🦲 Bob shows 👩🏾 Alice his proof of invitation, but uses Alice's username
+        const tryToAdmitBob = () => {
+          alice.team.admitMember(proofOfInvitation, bob.user.keys, alice.user.userName)
+        }
+
+        // 👎 But the invitation is rejected because the username is not unique
+        expect(tryToAdmitBob).toThrowError('Username is not unique within the team.')
+
+        // ❌ 👨🏻‍🦲 Bob is not on the team
+        expect(alice.team.has(bob.userId)).toBe(false)
+      })
+
+      it("won't accept proof of invitation with a userId that is not unique", () => {
+        const { alice, eve } = setup('alice', { user: 'eve', member: false })
+
+        // 👩🏾 Alice invites 🦹‍♀️ Eve by sending her a random secret key
+        const { seed } = alice.team.inviteMember()
+
+        // 🦹‍♀️ Eve accepts the invitation
+        const proofOfInvitation = generateProof(seed)
+
+        // 🦹‍♀️ Eve prepares keys using Alice's userId
+        const keysWithAliceUserId = {
+          ...eve.user.keys,
+          name: alice.userId,
+        }
+
+        // 🦹‍♀️ Eve shows 👩🏾 Alice her proof of invitation, but uses Alice's userId
+        const tryToAdmitEve = () => {
+          alice.team.admitMember(proofOfInvitation, keysWithAliceUserId, eve.user.userName)
+        }
+
+        // 👎 But the invitation is rejected because the userId is not unique
+        expect(tryToAdmitEve).toThrowError('userId is not unique within the team.')
+
+        // ❌ 🦹‍♀️ Eve is not on the team
+        expect(alice.team.has(eve.userId)).toBe(false)
+        expect(
+          alice.team.state.members.filter(({ userId }) => userId === alice.userId)
+        ).toHaveLength(1)
+        expect(alice.team.members(alice.userId).userName === alice.userName).toBe(true)
+      })
+
       describe('devices', () => {
         it('creates and accepts an invitation for a device', () => {
           const { alice: aliceLaptop } = setup('alice')
diff --git a/packages/auth/src/team/validate.ts b/packages/auth/src/team/validate.ts
index 33ebc884..b65232d5 100644
--- a/packages/auth/src/team/validate.ts
+++ b/packages/auth/src/team/validate.ts
@@ -112,6 +112,30 @@ const validators: TeamStateValidatorSet = {
     }
     return VALID
   },
+
+  /** Check if userId and userName are not used by any other member within the team */
+  uniqueUserNameAndId(...args) {
+    const [previousState, link] = args
+    if (link.body.type === 'ADMIT_MEMBER') {
+      const { userName, memberKeys } = link.body.payload
+
+      const memberWithSameUserId = previousState.members.find(
+        member => member.userId === memberKeys.name
+      )
+      if (memberWithSameUserId !== undefined) {
+        return fail('userId is not unique within the team.', ...args)
+      }
+
+      const memberWithSameUserName = previousState.members.find(
+        member => member.userName.toLowerCase() === userName.toLowerCase()
+      )
+
+      if (memberWithSameUserName !== undefined) {
+        return fail('Username is not unique within the team.', ...args)
+      }
+    }
+    return VALID
+  },
 }
 
 const fail = (message: string, previousState: TeamState, link: TeamLink) => {