Incorrect: Sometimes I like to be with one or two friends, sometimes I like to be with a large group of friends.
 

There are five ways to correct this run-on:
 

1. Make two sentences

Although your sentences will be grammatically correct, this is not the best way to solve the problem. It creates two choppy simple sentences instead of one complex one.

Sometimes I like to be with one or two friends. Sometimes I like to be with a large group of friends.

2. Use a coordinating conjunction (the word 'FANBOYS’ can help you remember the coordinating conjunctions: for, and, nor, but, or, yet, so)

Sometimes I like to be with one or two friends, but sometimes I like to be with a large group of friends.

3. Use a semicolon

You can use a semicolon between two closely related sentences. Be careful not to overuse this punctuation mark.

Sometimes I like to be with one or two friends; sometimes I like to be with a large group of friends.

4. Use a transitional adverb (however, therefore, consequently, etc.)

Be careful of the punctuation in this construction. Subject + verb; transitional adverb, subject + verb.

Sometimes I like to be with one or two friends; however, sometimes I like to be with a large group of friends.

5. Make one of the clauses a dependent clause

Although sometimes I like to be with one or two friends, sometimes I like to be with a large group of friends.