Updated in Telligent Evolution 5.6

Add an existing user account to an existing group.

EndpointPOSTapi.ashx/v2/groups/{groupid}/members/users.xml (or .json)
RequestGroupUserCreateRequest
GroupId Required GroupId
UserId Required UserId
GroupMembershipType Optional Types include: Owner, Manager, Member, PendingMember. Default if not specified is Member.
Example (C#)
var webClient = new WebClient();

// replace the "admin" and "Admin's API key" with your valid user and apikey!
// http://telligent.com/community/developers/w/wiki/how-to-generate-an-api-key.aspx
var adminKey = String.Format("{0}:{1}", "Admin's API Key", "admin");
var adminKeyBase64 = Convert.ToBase64String(Encoding.UTF8.GetBytes(adminKey));

webClient.Headers.Add("Rest-User-Token", adminKeyBase64);

var postUrl = "http://mysite.com/api.ashx/v2/groups/3/members/users.xml"; // or .json
var data = "<Request><UserId>2100</UserId></Request>";

webClient.UploadData(postUrl, "POST", Encoding.ASCII.GetBytes(data));
Example Response (XML)
<?xml version="1.0" encoding="utf-8"?>
<Response>
  <Info />

  <Warnings />
  <Errors />
  <User>
    <Id>2100</Id>
    <AvatarUrl>http://mysite.com/utility/anonymous.gif</AvatarUrl>

    <DisplayName>sample-data5e65a</DisplayName>
    <ProfileUrl>http://mysite.com/members/admin/default.aspx</ProfileUrl>
    <Username>admin</Username>
    <MembershipType>Member</MembershipType>

  </User>
</Response>
Example Response (JSON)
{
  "User": {
    "MembershipType": "Member",
    "AvatarUrl": "http://mysite.com/utility/anonymous.gif",
    "DisplayName": "sample-data5e65a",
    "ProfileUrl": "http://mysite.com/members/admin/default.aspx",
    "Username": "admin",
    "CurrentStatus": null,
    "Id": 2100
  },
  "Info": [],
  "Warnings": [],
  "Errors": []
}