Hi in your SPQuery please remove <Query> tag, also please escape character to remove empty space in your query.

    
`
query.Query =
        @"<Where><Or><Membership Type='CurrentUserGroups'><FieldRef Name='Users'/></Membership>" +
        "<Eq><FieldRef Name='Users'/><Value Type='Integer'><UserID /></Value></Eq></Or></Where>";`


you can also limit on results returned to one if you would like to by

query.RowLimit = 1;

If this fails, can you please debug can you please post the query value which you get? This should positively work.